Frontispi R~p roduud by permission of the Institution of Eltctn'cal Enginurs, from a painting by G. Harcourt, R.A., Faraday, I791- I867. ELECTRIC POWER Part !-History and Development.' 1.' ·NOTE Fig. 23.-132,000-volt " Grid " Transformln.g Station. ·Low Type.· Add ManufactUred and erected.by the English Electric Co ... ~td~. ,. 'f, • I p. 32, line 3. For anodes read rectifiers. p. 47, line 8. For casing by manilla paper impregnated with resin read casing by sulphite process wood~pulp· paper . impregnated with a black bituminous compound.· · · p. 47, line 34. For. Hockstadt~r .read Hdchstadter. (199&7) Wt.3315,3048 2\JilO 3j34 Hw. G.304 SCIENCE .MUSEUM SOUTH KENSINGTON HANDBOOK OF THE COLLECTIONS I~;LUSTRATING ELECTRICAL ENGINEci\.l.l~u I. ELECTRIC POWER. By W. T. O'DEA, B.Sc. Part I.-History and Development Crown Copyright Reserved LONDON PUBLISHED BY HIS MAJESTY'S STATIONERY OFFICE To be purchased directly from H.M. STATIONERY OFFICE at the following addreuea Adaatral House, Kingsway, London, W.C.:; no, George Street, Edinburgh 2. York Street, Manchester r ; 11 St. Andrew's Creaeent, Cardiff 1 s, Donegall Square West, Belfaat or through any Book.aeller 1933 Price 21. oJ. net CONTENTS PAGE PREFACE 4 INTRODUCTION 5 GENERATION 7 MoTORS 19 TRANSFORMATION AND CONVERSION - 25 SwiTCHGEAR AND PRoTECTIVE DEVICES 35 TRANSMISSION AND DISTRIBUTION - # INDUSTRIAL ELECTRICAL MEASUREMENTS - ss STORAGE 6s APPLICATIONS OF ELECTRICITY 69 INDEX - 77 LIST OF ILLUSTRATIONS FACING PAGE Portrait of Michael Faraday -Frontispiece Faraday's Magnet and Disc, 1831 Fig. I Woolrich Industrial Generator, 1844 - Fig.2 6 Pacinotti's Machine, x86o Fig. 3 Holmes' Magneto-electric Machine, 1867 - Fig. 4 7 Wilde's Dynamo, 1866 Fig. S 10 W. von Siemens' Dynamo, x867 Fig. 6 Ferranti-Thompson Alternators, x882-g Fig. 7 Mordey Inductor Alternator and Victoria Motor, 1888- Fig. 8 II Parsons' Steam Turbine and Dynamo, x884 - Fig. 9 Modem Turbo-alternator Set - - Fig. xo 14 Modem Vertical Waterwheel Alternators - Tesla Induction Motor, 1887-8 Faraday's Induction Ring, 1831 -i!: :;l :: Gaulard and Gibbs Transformer, 1882 Fig, 14 21 Modem Transformer Windings- Fig. IS Rotary Converter (Modem) Fig. 16 Supervisory Control Board (Modem) . Fig. 17 28 Steel Tank Mercury Arc Rectifier (Modem) "Clyde" Switch, 1903 - Metal-clad Switch Unit, 1905 - Modem Metal-clad Switch Unit r~:~l -Fig. 21 :: Modem Control Board - Fig. 22 3J " Low type " Outdoor Transforming Station Fig. 23 " High type " Outdoor Transforming Station Fig. 24 44 River Crossing Tower - Fig. 25 45 Ferranti Paper Insulated Cable, 1889- - Fig. 26 Edison Electrolytic Meter, x88o - - Fig. 27 54 Ayrton and Perry Polarised Iron Anuneter, x88o - Fig. 28 Ferranti-Wright Meter, 1890 - - Fig. 29 55 Stator of Large Winding Motor (Modem) - - Fig. 30 Transport of a Large Alternator Stator - Fig. 31 72 Heroult Electric Furnace - - Fig. 32 City and South London Railway Locomotive, 1890 - Fig. 33 73 3 PREFACE HE formation of a Museum of Science was first proposed by the Prince Consort after the Great Exhibition in 1851, and in 1857 T collections illustrating foods, animal products, examples of structures and building materials, and educational apparatus, were brought together and placed on exhibition in South Kensington. Subsequently many additions were made, including in I 884 the collection of machinery formed by the Commissioners of Patents, in 1900 the Maudslay Collection of machine tools and marine engine models, and in 1903 the Bennet Woodcroft Collection of engine models and portraits. Until 1899 the Art Collections and the Science and Engineering Collections together formed the South Kensington Museum, but in that year the name was changed to the Victoria and Albert Museum, which included both Collections until 1909, when it was restricted to the Art Collections ; those relating to Science and Technical Industry have since then formed the Science Museum. The aim of the Science Museum, with its Collections and Science Library, is to aid in the study of scientific and technical development, and to illustrate the applications of physical science to technical industry. This is effected by the informative display of objects, from modem as well as from past practice, so as to show the lines that their development has followed ; and when the centre block of the new Museum building is available it will be possible to represent current practice much more adequately than can now be done. Many of the exhibits are so arranged that they can be operated by visitors or demonstrated to them ; others have been sectioned so that the internal structure can be clearly seen ; and detailed descriptive labels are placed by every object. The Electrical Engineering Collection has been augmented from time to time by loans and gifts from many sources, including many scientific and technical institutions, industrial firms, and also private individuals. Recently the Institution of Electrical Engineers con tributed a large collection of objects of historical value, and many of those which were exhibited at the Faraday Centenary Exhibition have since been received. 4 INTRODUCTION HE industrial revolution was made practicable by advances in the generation and control of power. The conversion T of the latent energy of steam, water under a head, oil, or gas, into mechanical motion made possible the rapid mechanisation of industry, but the economic utilisation of these media is influenced considerably by the scale upon which the conversion takes place. Large units can be built on favourable sites and can be run with high efficiency, but the transmission of the resultant power to the various consuming points, which may be widely separated, can be achieved economically only by the medium of electricity. Electric power is . characterised by the ease and flexibility with which it may be both transmitted and applied to numerous consuming points differing greatly in their requirements. The result is that, while less than a century ago the only applications of electricity were the electric telegraph and electro-plating, the art of the electrical engineer has now spread to almost every industry and into the home. The discovery by Volta, in 1799, that electricity could be produced by chemical action soon gave to investigators an implement which made possible extremely rapid progress in electrical research. The high static charges from the frictional machine gave way to the steady low-tension current of the voltaic cell as a source of supply, enabling Oersted, in July 182o, to establish the hitherto unsuspected relationship between magnetism and electricity. He discovered that a current, passing through a wire, exerted a force on a neighbouring magnet. Arago and Davy, in the same year, noted a similar effect on a piece of unmagnetised iron, and Schweigger, in 1821, found that the effect was increased by passing the current through several turns of wire. Ampere then conducted a series of experiments which demon· strated several of the laws of electro-magnetism. In 1821 Faraday discovered that a conductor carrying a current could be made to rotate round a magnetic pole, and Davy, in 1823, found that a pool of mercury, carrying a radial current, would rotate in a magnetic field. • The electromagnet resulted from investigations by Sturgeon in 1825, and, in 1828, Joseph Henry, realising the significance of the number of ampere-turns, produced a greatly improved electromagnet wound with several layers of wire. Three years later the discovery by Faraday of the fundamental principles of the magneto-electric machine and the transformer inaugurated a period of development which raised electrical engineering to a position of universal importance. 5 [By permissiotJ of the Museum m1d A rt Gallery Committee of the Corporation of Birmingham . FIG. I.-Faraday's M agnet and Disc, I8JI. FIG. 2.-Woolrich Electro-Magnetic G enerator, 1844. Tofoc• />06• 7.) FIG. 3.-Pacinotti's Machine, 1860. F IG . 4.- Ho!mes' Magneto-electric Machine, 1867. NoTE.-ln till folluuing chapters, where reference is made to notable objecu which are represented in till Museum Collections by originals or replicas, an asterisk • has been inserted in till text. ELECTRICAL ENGINEERING GENERATION ICHAEL FARADAY, having discovered that a current could be induced in a conductor by moving it across the face of a M magnet constructed the first magneto-electric machine in I8JI 1 (Fig. 1 ). When a copper disc was rotated between the poles of a large permanent magnet he found that a deflection could be recorded on a galvanometer connected by spring contacts to the axis and periphery of the disc. This fundamental discovery was followed in the same year by an important machine by Pixii, consisting of a permanent magnet rotated in front of two iron-cored bobbins, thereby inducing an alternating electro-motive force in the bobbins. Ampere suggested the addition of a commutator to Pixii's machine in order that the current should be uni-directional but, with only two bobbins and one magnet, the electro-motive force (e.m.f.) generated by such simple machines was of far too pulsating a nature to satisfy the demand for steady currents created by the introduction of electro plating by Jacobi in I8J8. Stohrer, in 1843• constructed a multi-polar machine employing three compound horseshoe magnets above which six coils rotated, and in I 844 Woolrich constructed one of the first effective industrial electric generators for the electro-plating works of Messrs. Prime, of Birmingham (Fig. 2). It consisted of a heavy oak framework on which were mounted four compound horseshoe magnets. The armature, which rotated between the poles, contained eight bobbins from which a fairly steady current could be obtained. In 1850 Professor Nollet, of Brussels, commenced the design of a powerful magneto-electric machine with a view to decomposing water and procuring oxygen and hydrogen for lime-light. He formed a company to exploit these machines in· r8sJ, but, as regards the com mercial production of lime-light, the experiments were a failure. The Nollet machine, however, was improved by Holmes who, adding a commutator to a large multipolar machine, used this in conjunction with the arc light. The intense light given by the carbon arc had first been demonstrated by Sir Humphrey Davy in 1809, using a battery of 2,000 double voltaic plates, each four inches square, as a source of current. In December 1858 the South Foreland lighthouse was lit by an arc supplied by one of the Holmes machines. Faraday, having been sent to inspect the experimental machine built by Holmes in 1857, reported upon it very favourably, and it was largely through Faraday's recommendation that the machines were adopted by Trinity House. The machines installed at South Foreland contained sixty compound horseshoe magnets and a hundred and sixty coils mounted on two 7 wheels each about nine feet in diameter. Each machine absorbed 2'75 horse-power. In 1859 the Compagnie de !'Alliance commenced to make similar machines, but without the commutator, for lighthouse work in France, and Holmes in his later machines, such as that used at the Souter Point lighthouse in x87x and built in x867 • (Fig. 4) also dispensed with the commutator. The limitations of machines employing permanent magnets for excitation were, however, too great for much further progress to be made in economical output. Electromagnets had been employed in the primitive machines of Sturgeon, I8J2,1 Page, I837, and others, and, in 1845, Wheatstone patented the substitution of electromagnets in machines for telegraphic purposes. Three years later Brett passed current from the armature of a machine through a coil surrounding the permanent magnet in order to increase the strength of the latter. Jedlik, of Budapesth, constructed a uni-polar dynamo which was self-exciting, in x861, but the importance of this development did not appear to be appreciated at the time. In 1863 Wilde • patented his first machine in which a smaller machine or a battery was used to produce the exciting current (Fig. 5). Wilde's machines gave currents powerful enough for use in the commercial electro-deposition of copper, and his researches brought him very near to the full realisation of the principle of self excitation. Thus, in March x866, in a paper contributed by Wilde and read by Faraday before the Royal Society, the statement is made that 11 an indefinitely small amount of magnetism, or dynamic electricity, is capable of inducing an indefinitely large amount of magnetism. And again, that an indefinitely small amount of dynamic electricity, or of magnetism, is capable of evolving an indefinitely large amount of dynamic electricity" given the expenditure of mechanical power. He did not, however, get as far as building a true self-exciting machine. S. A. Varley, Werner von Seimens, and Sir Charles Wheatstone, in I 866-7, published particulars of their self-exciting machines in which the field was built up from the residual magnetism in the iron of the magnetic system. The residual magnetism remains for an indefinite period after the supply to the field windings has been interrupted, the frame of the machine acting in effect as a weak permanent magnet. If the armature of the machine is rotated in this weak field a small current is induced in the conductors, and this current can be passed through the field windings to strengthen the field. In this stronger field a larger current is induced in the armature and thus, by a pro gressive action, a strong fully excited field can be built up from the weak residual magnetism. Varley patented his machine • in x866, but afterwards let the patent lapse. Siemens communicated the principle to the Academy of Science at Berlin in January x867 (see Fig. 6), and in the following month Wheatstone described his machine • to the Royal Society in London on the same evening as C. W. Siemens described the discovery made by his brother in Germany. From his subsequent activities it would appear that von Siemens did most to 8 develop practically the discovery of self-excitation, and much of the credit for the universal adoption of this important principle is due to him. In xBss Dr. Werner von Siemens invented the shuttle armature, consisting of a cylinder of iron with two longitudinal grooves in which a coil of insulated wire was wound. The ends of the wire were attached to commutator segments, and the armature was rotated between magnet poles which were curved in order to embrace it closely and improve the magnetic efficiency. In x86o Dr. Antonio Pacinotti made a machine • using electromagnets and a ring armature (Fig. 3), but the importance of the latter seems not to have been appreciated generally until it was re-introduced by Z. T. Gramme in 1870. The Gramme machine was a great advance on those previously constructed and did much to increase the practicability of the application of electricity to industry. In the ring annature an endless spiral of insulated wire is wound on to an iron ring and divided into several sections connected to the commutator ; this gives a much steadier current than the shuttle armature, 'but that part of the winding which lies on the inside of the ring is idle, and, therefore, a waste of copper, and the threading of the coil through the ring is a troublesome process, particularly if the conductor section is large. In 1873 von Hefner Alteneck introduced the drum annature in· which the whole of the windings are on the outside of a cylindrical core, as in the shuttle, but are divided into sections to give the steady current obtainable from the ring type. For some years the ring and drum annatures were about equally popular, but the elimination of idle copper and the ease with which coils may be pre-fonned and slipped into position in the annature slots has led to the universal adoption of the drum construction. The early objections to drum annatures were due mainly to manufacturing difficulties, as it was common practice to bind the windings on to the outside of the core and the use of slots was looked upon with considerable misgivings. The adoption of the toothed annature (the teeth at first being known as "Pacinotti projections" after the inventor who used them in his machine of x86o) removed this disadvantage. The improvement in the efficiency of the machines constructed during these early years may be gauged from the following test results obtained, in a competitive trial of machines for lighthouse supply, by Trinity House in 1876-7. At that time standardised efficiency tests were unknown. Light produced per horse·power expended, Type of machine in standard candles Condensed Diffused Holmes magneto-electric • , 476 476 Alliance magneto-electric , . 543 543 Gramme (early type) 1,257 758 Siemens, large •• 1,512 9Il Siemens, small 2,o8o 1,254 9 Dr. John Hopkinson devised a back-to-hack method of taking efficiency tests in 1886 and obtained the following results : Siemens machine (about 15 h.p.), 88 per cent. efficiency. Edison-Hopkinson dynamo, 1883 (about 15 h.p.), 85 per cent. efficiency. Edison-Hopkinson dynamo, later (35 kW.), 93'2 per cent. efficiency. These efficiencies, however, were only obtained by designing on generous lines which would be entirely uneconomic if applied to modem machines. · The direct-current machine received most attention until the period x877-85, when several factors emerged in favour of the alternator. The Jablochkoff candle, in which the carbons were of the same size, placed side by side, was introduced in 1877 and required an alternating supply. Transformer*fed distribution schemes developed from two notable inventions, that of Gaulard and Gibbs, who patented the use of a number of induction coils in series for high voltage transmission, in 1882 ; and that of Zipemowski, Deri, and Bl~thy who drew attention to the advantages of the parallel connection of transformers in x88s. Hopkinson expounded the principles underlying the parallel running of alternators in x882-4, and further progress in the use of alternators became possible. The development of the direct*current machine, however, proceeded steadily. In x881 Edison constructed a public supply station, to serve consumers using his newly-invented carbon filament lamp, at Pearl Street, New York. He employed Edison D.C. dynamos direct coupled to high-speed steam engines, and in x88z the Holbom Viaduct station in London, employing the Edison system, followed. The magnetic circuit of the Edison dynamo was greatly improved in 1883 by the mathematical and experimental investigations of Dr. John Hopkinson, to whom much is due for the application of scientific methods to the principles of design. He showed the superiority of single magnet limbs of moderate length over the multiple attenuated limbs favoured by Edison, and the advantages of the small air gap and a generous supply of iron in the armature core combined with the elimination of clamping bolts passing through the armature laminations. The Edison machines, and those of Brush built in I 883, employed laminated cores to reduce eddy current losses, thereby increasing greatly in efficiency. Forbes, in x883, introduced carbon brushes* which later superseded the copper wire or leaf previously employed. Breguet found that the position of the brushes with relation to the magnet poles affected commutation, and Hopkinson and Kapp, in x886, made investigations contributing much towards a greater understanding of the problems of the magnetic circuit. The advantages of the enclosed ring-yoke armature were appreciated, and machines of this type were constructed by Thomson, Kennedy, and Lahmeyer in 1886-7. The ring yoke, universally adopted since, had actually been employed by Gramme in the exciter of an earlier machine of about x88x. IO [To face Pag~ 10, • II• • Fie. 5--Wiide's Dynamo, 1866. fk 6.-0rig;n,J Self. FIG. 8.-:Mordey Inductor Alternator and Victoria Motor, 1888. The characteristics of shunt and series machines were fully investi gated, and in 1876 S. A. Varley patented the compound winding. He realised the advantages of regulation, but let his patent lapse. Afterwards, as a result of litigation which reached the House of Lords, it was the means of upsetting the patent of Brush, taken out in 1878. The development of armature windings, such as wave and lap windings, followed ; ventilation received attention ; interpoles were introduced to assist commutation ; improved materials were dis covered for insulation and for the magnetic circuit, and, combined with better mechanical design, led to an increase in efficiency and reliability. The modem direct current machine of large output may have a full load efficiency in excess of 96 per cent. The required voltage/load characteristic may be obtained by suitable compounding, and the temperature rise of the windings due to any specified load conditions may be estimated with reasonable accuracy. The construction may be open, drip-proof, or totally enclosed, while in the latter type water cooling of the ventilating air may be employed. Yokes may be of rolled, cast, or fabricated steel. Poles, or pole shoes, and the armature core are made of thin laminated sheets, insulated by paper or spraying, and compressed together. Slots may be skewed to eliminate tooth ripple and noise. Insulation is commonly of mica wrap in the slots with treated and impregnated tapes and cloths round the conductors. Hard fibre wedges retain the conductors in the slots. Wide face com mutators may be divided to reduce the effects of uneven wear, and machines are mounted on ample bearings attached to a rigid bedplate of cast or welded steel. The alternator, since 1877, has developed with even greater rapidity. Gramme • and Meritens, in France, Westinghouse, in America, Zipemowski, in Hungary, and Kapp, Mordey,• Forbes, Thompson, and Ferranti,• in England, among others, produced many types of single-phase alternators between 1878 and 1890. Most of these were of small capacity, although Ganz & Co. built Zipemowski alternators in fairly large sizes. Blathy, in x888, followed up the experimental work of Wilde (1868), Adams and Hopkinson (1882-4) by connecting two Zipemowski alternators of 6oo h.p. in parallel with two others of 150 h.p. at the Central Station, Rome. Ferranti, in 1889, designed a very large machine,• of the Ferranti~ Thompson type introduced in 188z,• for the Deptford Power Station of the London Electric Supply Corporation (Fig. 7). He decided on large independent units giving 100 amperes at Io,ooo volts owing to the many uncertainties in the paralleling of the machines of the period. The current was first carried by a rubber-insulated overhead cable which was later replaced by a paper-insulated underground cable joined in 20 feet lengths between Deptford and London. The attention of the scientific world was compelled by the magnitude of this pioneer undertaking which depended for success on the solution of many II technical problems. For years its history was punctuated by a series of breakdowns, and although immediate commercial success was not achieved, the courageous policy of the scheme was vindicated scientifi cally by later developments to which it contributed very valuable experience. Design, in these early years, was largely a matter of trial and error,· and the formulation of generally accepted rules was slow. Some of the divergencies of opinion were due to the doubtfulness of many of the materials available, and as a result of the later introduction of supplies made to known specifications, several early designs appeared re actionary. Thus, many designers, including Ferranti and Mordey, thought at one time that iron was undesirable in the armature of an alternator, and slotted armatures were considered by some to be objectionable. The poor permeability of high carbon steel largely justified such assumptions. The following table is given by Professor Walker to show the improvement in dynamo steels since 1893 : Loss in watts per lb. under standard Material conditions. (Maximum flux density Io,ooo lines per sq. em. at so cycles with iron o.s mm. thick) Dynamo steel, 1893 2'1 Good ordinary, 1914 1'7 Better quality, 1914 • . • • 1'3 Silicon steel (3 per cent. silicon) 0'9 Silicon steel (3·5 per cent. silicon) .. o·8 Silicon steel (4·8 per cent. silicon) (too o·s6 hard for general use). High self-induction with consequently poor regulation was thought to be desirable in machines for parallel running. Frequency was largely a matter of choice, several designers believing that high economical output ..depended to a great extent on speed. The Westing house and Lowrie-Parker alternators, both iron cored, ran at frequencies of 133 and So cycles per second respectively. Zipernowski, in order that small machines with few poles should not be required to run at excessive speeds, standardised a frequency of 42 cycles for his iron cored alternators. Ferranti's machines for Grosvenor Gallery ran at 8o cycles, while those for Deptford were designed for 68 cycles. The Mordey alternator worked at 100 cycles. In recent years the standardisation of frequency in this country, under the auspices of the Central Electricity Board, has cost more than ten million pounds. The efficiency of the early alternators may be gauged from the following test results : 8! h.p. Meritens magneto-electric machine (x88o) 68 per cent. II h.p. Siemens machine (x88x) 69 per cent. 26 h.p. Ferranti-Thomson machine (1882) 89 per cent. The machines, however, were very much larger than equivalent machines would be to-day, with consequent effect on the first cost or relative economic value. J2 Single-phase alternating current supply was handicapped by the. difficulty of starting motors. Except in the case of the A. C. commutator motor, introduced by Alexander Siemens in x884, it was necessary to bring motors up to speed by auxiliary means before switching on to an A.C. supply. The self-starting polyphase motor of Dobrowolsky, commercially introduced in x8gx, gave an impetus to schemes of alternating current distribution. The earliest means of producing polyphase currents was by the uneconomical method of coupling together, mechanically and electri cally, a number of single-phase generators. In 1887 Haselwander obtained the same result by the use of independent windings mechani cally spaced around a single armature, and the first Central Station generating three-phase currents was built to feed the Lauffen-Heilbronn project of 1892. In a two-phase generator there are two independent armature circuits, the coils of which occupy alternate groups of slots so arranged 0 that the e.m.fs. generated in each circuit differ in phase by gq • Each circuit experiences the same variations in magnetic flux, and therefore in e.m.f. induced, during each complete revolution of the armature ; but, as one circuit is mechanically spaced from the other by half the distance between poles the e.m.f. in one is zero when the other is at its maximum value, and the e.m.f. in the first circuit increases to the maximum value while that in the second is declining to zero. In a three-phase generator the phase difference is I20°, the e.m.fs. in the three circuits attaining successive maximum values each time the armature rotates through an angle equivalent to the distance between poles. The frequency of the alternations in voltage depends on the number of poles and the speed of rotation. In April x884, Sir Charles Parsons took out his first two patents for the turbo-alternator set • (Fig. 9). The design of a dynamo which would function satisfactorily at the high speed at which the turbine ran (x8,ooo r.p.m.), and therefore at high frequency, was almost as great a problem at that time as the design of the turbine. The development of the turbine and alternator, however, subsequent to his invention, has been such that the cost of generating electricity from steam in bulk is probably not more than half what it would have been otherwise, although the early experimental turbines were less efficient than steam engines. In 1887 a small turbine and generator, running at u,ooo r.p.m., was shown at the Newcastle Exhibition, and' a year later the Newcastle and District Lighting Co. purchased a 75 kW. set running at 4,8oo r.p.m. The original Parsons patents were for axial flow turbines, and, after some differences with his associates from whom he parted company, Parsons found himself unable to proceed with such designs owing to having lost control of the patents. He therefore devised the radial flow turbine, which, though not as promising as the earlier type, had been brought to an efficiency equal to that of a steam engine of similar capacity by 1892. Soon afterwards he was able to revert to the axial flow principle, ~d by the year 1899 IJ turbo plant rated at 1,000 kW. was being built, and in 1902 the first revolving-field high-speed alternator was built for the Newcastle on-Tyne Electric Supply Co., Ltd. C. E. L. Brown, of Brown, Boveri, is credited with having developed the non-salient pole rotor. In 1903-7 the size of turbo sets increased to a 4,000 kW. rating, and in 1912 a 25,000 kW. set was built for Chicago. The early problems encountered included the running of steel discs under stresses which could not then be calculated, and the difficulty of dealing with the great armature reactions experienced at such high speeds. The latter problem was ingeniously tackled by arranging that the position of the brushes should be altered by steam pressure under variations in the load, the variation of the strength of the field being counteracted by special windings. Trouble was experienced with the armature connections to the commutator owing to springing of the shaft, and Parsons was the first to introduce flexible connectors to overcome this. The modem turbo-alternator is a monument to the patience and inventive genius expended in over coming what at one time seemed almost insuperable difficulties. In a turbo-alternator set, where the turbine may drive the alternator rotor at a speed of 3,ooo or I,soo r.p.m. only two or four poles respectively are required to give a frequency of so cycles per second. Slow speed gas or steam engine sets, rotating at, say, 100 r.p.m., would require 6o poles to give the same frequency. The turbo alternator rotor is made small in diameter in order to keep down the peripheral speed and consequent centrifugal forces. In the case of the engine-driven alternator the slow rate of revolution allows a diameter of rotor several times as great as that of a turbo-alternator to be used without too high a peripheral speed being reached. In order to accommodate the large number of poles required, such a diameter is necessary. In addition the flywheel effect of a large diameter rotor is useful in correcting the irregular impulses given by a reciprocating engine. , In water-driven machines the speed of rotation does not approach the high speeds of steam turbine sets, being more nearly that of the engine drive. According to the type of water supply available, i.e. a large volume under a low head or a relatively low volume under a high head, the characteristics of the driving machine may alter with corre sponding effects on the design of the generator. It is often found convenient to mount the set vertically on pedestal bearings with the driving unit underneath (Fig. 1 I). C. E L. Brown constructed umbrella type units for low-head hydro-electric schemes in 1897· Gas, Diesel, or steam engines are rarely used as prime movers for sets rated above 1,500 kW., and all large central station units are driven by turbine sets or water power. The largest turbo-alternator installed in England up to the year 1932 was rated at 75,000 kW., and the highest directly generated voltage was JJ,OOO. In America a turbo· alternator rated at 16s,ooo kW. has been built, such large units fulfilling really exceptional requirements. The question of transport must be 14 [To faa page 14 Fw. 9.-0riginal Parsons' Stearn Turbine and Dynamo, 1884. [M•tropolitarr- Vickers Ekctrical Co., Ltd. Fxc. Io.-s1,250 k\V., 1,500 r.p.m., Turbo:Alternator Set. Tofon J>t1P 15] [.ll the more usual II,ooo-I3 1000 volts received little support until two 33-36 kV., 25,000 kW. Parsons machines were built for the North Metropolitan Electric Supply Company, incorporating a special concentric conductor principle in the windings. Where outgoing feeder lines work at the generation pressure of 33 kV. a saving is made on the cost of transformers, transformer losses, buildings, and switch gear, which more than compensates for the increased cost of the alternators, while it is claimed that short-circuit forces on the end connections are reduced and harmonics may be minimised in the high pressure machine. An increase in the use of such alternators may be expected, and it is possible that generators working at much higher pressures may be developed. The constructional improvements in direct current machines which have been described are, in general, applicable to alternator • practice. Turbo-alternators are often totally enclosed with a separate cooling system for the circulating air. Hydrogen-cooled sets have been built in which the efficiency is increased due to the reduced losses in driving the low density cooling medium through the machine. Ionisation also occurs at a higher voltage in hydrogen than in air. Conductors may be laminated to reduce eddy current losses and low loss transformer iron may be used in the core laminations. Rotors are of the non-salient pole type in high-speed machines, and special atten tion is devoted to the question of insulation which will not shrink in 15 consequence of the action of temperature and centrifugal force. l\tica and asbestos are commonly used for this purpose. Coils, after being baked and placed in position, are clamped under high pressure so that possible " breathing " of the coils is eliminated. The ends of coils outside the iron are blocked together to prevent movement due to short circuits, by spacers of hornbeam or other materials. The forces in a large turbine set are so large and complex that a period varying from half an hour upwards must usually elapse between starting up from cold and switching in on to the station busbars, and slow decelera tion is also essential. The efficiency of the alternator may be as high as 98 per cent. and that of the turbine about 82-87 per cent. One very important requirement in large rapidly rotating machines, early appreciated by Parsons, is accurate balancing. Several effective methods of ensuring symmetrical weight distribution have been introduced into manufacturing practice. Parsons was also responsible for the introduction of forced lubrication and also, by overspeed running before final erection, he expanded the discs to an extent sufficient to ensure that troubles from expansion, due to centrifugal forces, were unlikely to arise when the sets were put into actual service. It is most economical, within limits, to produce electric power from fuel in large quantities at a central point rather than have a number of small generating stations distributed wherever a consuming area may be situated. The power developed at a central station may have to be · transmitted over long distances, in which case it may be more economical to transmit, through the interposition of transforming stations, at a much higher voltage than that at which it is convenient to generate. The site of a power station may be remote from both the consumers and the fuel supply, or, if the prime mover is operated by water power, the source of such power may be removed from the consuming area by a long distance. The economies due to large scale generation and inherent site facilities justify the apparent isolation of the station in such cases. It is interesting to note, however, that the London Power Co. has recently constructed a very large station in the· central area of Battersea, a reversal of the policy inaugurated by Ferranti in the construction of the Deptford Station of x889. In the choice of a site for a station employing steam turbine-driven sets the chief factors to be considered in relation to remoteness from the consumer and fuel costs are, in order of relative importance : I. The amount of water available for the condensing plant. 2. Good transport facilities and foundations and low land values. In the case of water power stations the provision of standby fuel- driven plant for use during low-water periods must be considered. In Italy, during the winter of 1921-2, the water levels were reduced to such an extraordinary and unforeseen amount that factories had to be rationed as to the load they might consume, and considerable industrial dislocation resulted. The layout of power-station buildings is complicated by the necessity of providing against many seemingly remote contingencies, 16 as the importance of the continuity of supply under all circumstances must always be taken into account. Thus, in the fairly remote possi bility of a circuit breaker failing to clear a serious fault it is probable that burning oil or compound might spread through the affected portion of the switch-house. It is important that such a disaster should be confined within narrow limits, fireproofing including attention to the possibility of cable ducts allowing the fire to spread to non-affected areas. There has been a case where a burning jet of oil under pressure rose 6o feet from a turbine and ignited a wooden roof which finally collapsed with considerable consequent damage. The possibility of such eventualities makes it desirable, therefore, that the architect and electrical engineer should collaborate in order that accessibility, durability, and layout should be combined in those proportions which give the best economic balance. Where load conditions demand interconnected networks with several generating stations linked together, possibly by long trans mission lines, it is necessary to employ means other than voltage regulators in exciter field circuits or induction regulators in feeder circuits to ensure the delivery of a reasonably steady voltage to con sumers, as it is usually desirable that both the energy load and the wattless load should be divided in an appropriate manner between the various stations. Energy control is dependent on the regulation of the supply of the motive medium (steam, water, etc.) through throttle or valve governors, while the division of the wattless load can be arranged by regulation of the generator fields, but at the expense of irregularity in the busbar voltage. This irregularity can be corrected at the transformers if the latter are equipped with on-load tap-changing devices to vary the ratio between the primary and secondary turns. Frequency control is also particularly desirable on interconnected systems. It results in the simplification of metering, the correction of the tendency known as " load-snatching," easy re-synchronisation of disconnected units of a system, and constancy of the speed of connected synchronous motors. This latter feature is a valuable asset to certain industrial drives and has resulted in the wide employment of synchro nous electric clocks, which are also being used for driving recording meters and for registering with extreme accuracy the time periods in " period-demand " systems of charging for power supply. " Load-snatching "is a condition peculiar to the control of a number of units in parallel on a fluctuating load which the control engineer must allocate between various stations. The load is usually divided amongst the units, leaving one unit with" speed-control." This unit must take up temporary variations in the load by the admission, in a steam station, of more steam to the turbines on a rising load and less on a falling load. The admission of more steam tends to raise the speed of the turbine, this tendency being corrected by a slowing-down due to the imposition of more load. If the speed, and consequently the frequency, rises more than is necessary the control station tends to take load off the other stations in the group, and vice-versa. The 2-(261) 17 exercise of close control over the frequency of the various units elimi nates this undesirable interchanging of load. Such control may be semi or fully automatic, depending on the differential action between a synchronous motor driven by the supply to be controlled and an independently driven standard clock, the constancy of which may be ensured by piezo-electric or tuning-fork action. Any difference in speed between the clock and the motor is shown by the differential drive which may be used to actuate appropriate relays controlling the steam supply. The great advances which have been made in the practice of steam raising have been influenced considerably by the requirements of electrical power distributors. Both the pressure and temperature of the steam raised in a modem power station employing mechanical firing, water-tube boilers with solid forged drums, water-wall hearths, and mechanically improved draught, are far in advance of those formerly available. Following the erection of the North Tees Power Station using steam at 475 lbs. per square inch, by Metropolitan-Vickers in 1917, progress has been such that 8oo-1,200 lbs. per square inch steam pressure and temperatures up to 850° F. are now not uncommon, and higher figures have been reached in some installations. Coal and ash handling has been almost completely mechanised. It is, however, not within the scope of this handbook to deal in detail with steam-raising plant in which the great progress of the mechanical engineer has been fully utilised for the development of electric supply. J8 MOTORS The growth of electrical engineering is due to the ease with whlch power can be generated economically in bulk and transmitted to a large number of consuming points requiring little or no attention, combined with the flexibility with which the power can be utilised. The motor provides an excellent example of these advantages. The mechanical work done in driving the central generator is distributed by electrical means, subject to a reduction due to transmission and efficiency losses, among a number of scattered applications by means of motors, each of which may have characteristics specially suited to its function. These characteristics may differ greatly from those of the generator. Direct Current Motors. The direct current motor is more flexible in operation than the alternating current types, but alternating currents are at present much more convenient for transmission purposes. For this reason it is common to find crane motors, requiring characteristics peculiar to direct current motors, being run off alternating current mains by the interposition of mercury arc or metal rectifiers. Some of the earliest attempts to convert electricity into motive power were the primitive electro-magnetic engines in which successively excited electro-magnets attracted iron pistons attached to a crankshaft as in the steam engine. These were soon superseded by machines with rotating armatures. Direct current motors are, in general, of three types :-series, shunt, and compound wound, depending on the method of connection of the field magnet windings. They developed concurrently with the D.C. generator, the fact that the latter could be used as a motor being appreciated by Lenz in r838. The series motor has a large starting torque and is therefore suitable for starting under load conditions, as in applications to traction schemes. Its speed decreases with load. The shunt motor has a small starting torque, but its speed under variable load conditions is nearly constant. The compound-wound machine combines the characteristics of both. In all three types the speed may be varied by altering the field current. These motors are self-starting. from rest through a starting switch connected in the supply circuit. The armature of a direct current motor, when running, generates a back e.m.f. which is slightly less than the supply voltage. The resistance of the armature is low, but the current taken is due only to the difference between the supply volts and this back e.m.f. A stationary machine must therefore be supplied initially with only a fraction of the full supply voltage in order to avoid an excessive current passing through the armature. This is accomplished by means of a starting switch which gradually cuts out an appropriate resistance, 19 placed in series with the armature, as the motor gathers speed. Such switches are hand-operated on small motors, but on a large machine dealing with very heavy currents the timing of the moments when further resistance may be cut out and the quick suppression of arcing is of considerable importance, and electrically-operated contractors may do this automatically after the supply has once been switched on. "Over-load" and "No-volt" releases are normally fitted to trip the switch back to the starting position in the event of a serious over- . loading of the motor or a failure of the supply voltage. For traction motors drum-type controllers are used which, as they are notched round, first cut out resistances in the motor circuits, two or more motors being connected in series, and then, at a later stage, connect the motors in parallel. One master controller operates all the others in the case of an electric train divided into several motor coaches. AlternatinJ1 current motors may be classified as follows : I. Synchronous Motors. The early development of this type of machine was due to Wilde, Adams, and Mordey, and much is due to Hopkinson, who in 1883 developed the theory underlying their operation. In 1868 Wilde described parallel working and synchronous action of generators and so nearly obtained synchronous motor action that it is extraordinary he should have missed it. U an alternating current 'generator which, when driven by mechanical means would be suitable for paralleling on to a supply circuit, is first run up to the synchronous speed and is then connected to the supply without further mechanical aid, it will continue to run as a motor. Such a machine is not self-starting, as it must be run up to synchronous speed before being connected to the supply, but as this initial speed is attained only under no-load conditions a relatively small auxiliary motor is sufficient to perform the starting operation. Alternatively the synchronous motor may be so wound and connected with its exciter that it may be started as an induction motor and then, when the necessary speed has been obtained, may be converted to synchronous working by a switching device. The speed of a synchronous motor is constant under all load conditions except that if such a motor is too heavily overloaded it will fall out of step with the supply frequency and stop. The synchronous speed is determined by the supply frequency and the number of poles on the motor. Thus on a so cycle supply an eight pole motor would run at 750 r.p.m.; a ten pole motor at 6oo ; a twelve pole motor at soo, etc. One advantage of the synchronous motor is that it can be run at a leading power factor, thu's compensating for a lagging power factor due to an induction motor load. The capacity of supply mains, switchgear and generating plant depends to a great extent on the power factor of the load they carry, and therefore, in order to reduce capital expenditure on a system, it is often economical to install synchronous motors. JO FIG. 12.-Experimental Induction Motor by Tesla, 1887-8. [From the original at tlu Rayal/nllituticm of Gr•at Britain. FIG. 13.-Faraday's Induction Ring, 1831. To/tJU ~· 21.] FIG. 14.-Gaulard and Gibbs' Transformer, 1882. [M•tropolitart-Vidtn-r El•elrical Co., Ltd. FIG. 15.-3o,ooo kVA., 3-phase, 6,6oo-u,ooo volt Transformer removed from tank. Tapping connections and tap-change bridging choke coils are shown. 2. Inducti 1. Transformation, or change from one voltage to another without varying the system of supply. Thus, while generation may be most economical at a voltage of say, u,ooo, transmission over a long distance may determine a trans mission voltage of I 32,ooo at which the current to be carried is only one-twelfth of that which would be required at II,ooo volts. A greatly decreased section of conductor is then necessary for the same trans mission losses, which vary with the square of the current. The cost of transforming equipment, extra insulation, and higher but more lightly loaded supports may therefore be more than offset by the reduced expenditure on conductors. For secondary distribution lines 66,ooo, JJ,ooo, or u,ooo volts may be most economical, and for actual application a voltage of soo or 250 may be required. For chemical works a further transformation to thousands of amperes at an exceedingly low voltage is often necessary. Testing transformers have been built to give several hundred thousand amperes under short circuit conditions. 2. Phase transformation, or alteration of the number of phases in an alternating current system. It may be required to interconnect, say an existing two-phase system with an adjoining three-phase supply or to vary the number of phases of an electric locomotive drive in order to obtain greater speed control. Mercury arc rectifiers give a smoother D.C. supply from multiphase alternating currents. 3· Change of frequency of an alternating current supply. This may be necessary in order to interconnect two or more systems in which the frequency differs or to supply a traction scheme, in which low frequency may be an advantage, from a general distribution systetn at a higher frequency. 4· Conversion from alternating to direct current or vice versa. THE STATIC TRANSFORMER Faraday, in I8JI, discovered that if two separate insulated coils were wound on to an iron ring • (Fig. 13) the making, breaking or altering in value of the current in one coil caused a transitory current 25 to flow in the other. From this discovery the evolution of the induction coil proceeded, and it was found that a definite relationship existed between the voltage applied to the primary coil and that induced in the secondary coil, depending on the relation between the number of turns on each coil. In 1837 Masson improved the efficiency of the induction coil by laminating the core. An alternating current was first applied to an induction coil by Sir W. Groves, and Jablochkoff, in 1877, made use of independent coils as transformers in his system of arc lighting. As an alternating current is continuously fluctuating, similar fluctuations are experienced in the secondary coil at a voltage proportional to the ratio of the number of turns on each coil. If the secondary coil is loaded the power supplied to the primary coil will be equal to that taken from the secondary coil plus an amount which is dissipated in losses in the transformer itself. Thus, with a secondary output of 2 amperes at Io,ooo volts and ten times as many turns on the secondary coil as on the primary, the input to the primary would be at I ,ooo volts, the current taken being 20 amperes plus an extra amount to provide for losses. In 1882 Gaulard and Gibbs patented a system using transformers • (Fig. 14), or " secondary generators " as they were then called, in series, and an experimental lighting system was installed on the Metropolitan Railway. An interesting sidelight on the series trans former scheme and the lack of knowledge of distribution problems was provided by the experiences with the Grosvenor Gallery Station in 188s. The directors of the gallery, which was then at the height of its fame as a centre for art and culture, decided to light it electrically, and the owners of several neighbouring buildings wished to be connected. Legal . difficulties made underground distribution impracticable, and it was decided to run overhead wires, which, at lamp voltage, would have been very heavy to carry the necessary current. It was decided therefore to employ the series transformer system of Gaulard and Gibbs at about I ,zoo volts pressure. The customers were connected by a ring main, but it was found that induction was so strong that the Telephone Company's West End Exchange became unworkable and the noise was transmitted all over the Metropolitan area. Eventually, after consultation with Dr. John Hopkinson, the main was doubled and the old main cut at its furthest points so that the currents returned by an adjacent circuit and the induction was reduced to negligible proportions. The great advantages of working transformers in parallel were demonstrated at Budapesth in 1885 by Zipernowsky, Deri, and Blathy, who also made improvements in design. Stanley also used parallel working, independently, in America a year later. The clos~d magnetic circuit employed by Faraday was again used by Varley ~~ 1856 in an induction coil and was re-introduced by Zipernowsky, Den, and Blathy, whose transformers were very similar in appearance to Faraday's original induction ring, except that the iron was wound round the coils. Ferranti, who quickly appreciated the advantages of 26 parallel working and the constructional benefits of winding the coils round the core, took out his first patents in 1885 in conjunction with Addenbrooke, and in 1891 he designed transformers • of 150 h.p. capacity and Io,ooo volts working for the London Electric Supply Corporation. In 1891 Oelschager and Gorges introduced the first three-phase transformers in association with C. E. L. Brown, of Brown, Boveri. They had three cores situated at 120° to each other, connected above and below by a round yoke. The double concentric winding was used for the first time in this design. The losses occurring in transformer cores (as distinct from copper losses in the windings) are made up of hysteresis and eddy current losses. Both of these are much lower in the silicon steel introduced by Sir Robert Hadfield in 1903 • than in the transformer iron used before its adoption (see table on page 12). Before this date the phenomenon of " ageing," involving an increase of at least 3o-5o per cent. in the core losses with time, necessitated periodic dismantling of transformers for annealing of the cores, except in the case of some special transformer irons in which, however, the hysteresis and eddy current losses were initially great. The introduction of silicon steel had therefore a very great effect in improving the efficiency of transformers and stabilising their performance. The electrical resistance of a steel containing r8 per cent. or more of silicon is increased, while the permeability, for inductions below 14,ooo lines per sq. em., is increased also. Silicon steel is also used for the armatures of machines, but, where highly saturated teeth may be required, it must be remembered that the permeability is relatively low at very high inductions. In addition, the hardness and brittleness of a high silicon steel have led to the employment of sheets of lower silicon content as a compromise in certain cases. Transformer cores are of two main types-the core type in which a laminated iron core is surrounded by the coils, and the shell type in which the core is, in effect, wound round the coil. The advantage of the former type is greater accessibility of the coils and better cooling, but the shell type usually requires less copper and less magnetising current. For polyphase transformers the core type of construction is usually more convenient and is becoming more common also in single· phase transformers. A combination of the two types, known as the radial type, is also employed. Cores are built up of metal stampings interleaved at joints. .. In order to reduce leakage flux to a minimum the primary and secondary windings are usually divided up into alternate sections. Insulation is graded so that destructive line surges, which affect mainly the first few turns, may he resisted, while a type of " non-resonating '' transformer has been introduced in which it is claimed that by placing metallic shields, properly proportioned, outside the windings the voltage distribution in the case of a surge is made as uniform as possible. Considerations of cost militate against such designs. The windings of all transformers must be solidly clamped to overcome short-circuit 27 forces. In certain cases duplicate windings are so arranged with high reactance between them that, when connected to two separate busbars or systems, they are equivalent to a bus-tie reactor. Multi-winding transformers for operation on several distinct output voltages at the same time have been constructed. As an example, the continental firms, Oerlikon and Brown, Boveri, made transformers in 1932 for a three-phase 10'5 kV. primary supply having secondary windings each of 35,ooo kVA. capacity for 48, u6, and 145 kV. Although the full load efficiency of a large unit is commonly between 98· 5 and 99'7 per cent. the power to be dissipated as heat from the transformer is considerable. The iron losses in the multi-winding transformers described above are as much as 250 kW. For very small sizes ordinary air cooling is sufficient, but most transformers are contained in oil-filled steel tanks provided with radiating fins or pipes. The oil is also an insulator. C. E. L. Brown designed three-phase 86/zs,ooo volt transformers for the Lauffen-Frankfort scheme of 1891 in which oil cooling was first employed. In some cases cooling water is circulated through copper pipes placed in the oil at the top of the tank, or the oil is circulated through an external cooler. In order to prevent the possibility of breakdown in auxiliary cooling apparatus, which might cause serious damage, a number of large transformers have been built which employ natural cooling, the necessary surface being obtained by erecting detachable radiators on the tank or in outside banks. Forced air cooling is sometimes applied to such radiators, particularly where the units are very large. Unforced natural cooling is usually obtainable for sizes up to about 3o,oo? kVA. The oil in the tank is commonly under a small pressure created by mounting an expansion chamber on top of the tank, and silica gel or calcium chloride " breathers " are usually fitted to such conservators. In certain cases this chamber is dispensed with and the top portion of the tank is filled with an inert gas (nitrogen). It is usual to ship a transformer complete with oil, as the windings must be carefu1ly dried out at the works before the oil is inserted, and it is advisable that they should not afterwards be exposed to the atmosphere. Another method which has been used with success is the shipment of transformers in dry air under pressure. Insulators may be of the bakelite condenser type for indoor service, but for outdoor installations porcelain or stoneware rain shields must be provided. For the lower voltages plain porcelain bushings are used. One of the chief factors limiting the size of a transformer is the question of transport, and rail and site facilities must be carefully studied. In England loads up to 1 so tons may now be carried, and railroad track limitations are mitigated as an elongated shape is usually convenient in a transformer. The largest transformer built in England up to the end of 1932 was rated at 93,750 kVA. 28 [To face page 28. [Metropolitan-Vickers Electrical Co., Ltd. FIG. 16.-875 kW., I,ooo r.p.m., Rotary Converter. I I 1 I i llrll 'II "IllII •rtt• :~ ~::t duuil lUI II II "[Metropolitan-Vickers Electriull Co., Lid. FIG. 17.-Supervisory Control Board: Central Argentine Railway. The diagram contains signal lampe and operating keys for supervisory control of converter aubstationa and track sectioning cabLns. To faa J>og• 29.] [British T/wmscm-Huustcm Co., Ltd. FIG. xS.-x,soo k\V., x,soo-volt Steel Tank Mercury Arc Rectifier. It is usual to keep at least one complete unit as a standby in case of breakdown in any installation. In this respect it is interesting to note that four single-phase units of moderate size usually cost about as much as two three-phase units, and three of the latter are much cheaper than seven single-phase transformers, so that the three-phase type is becoming more common, particularly as they occupy less floor space and require less cable. The regulation of a transformer, that is, the rise in voltage from full load to no load on the secondary side, must be confined within narrow limits, and it is common to fit a switching device which introduces or cuts out turns so that the voltage delivered remains more or less constant under all load conditions. On-load tap changing introduces complica tions in design, but it is now almost standard practice in units supplying large networks. From the point of both first cost and maintenance it has advantages over other methods of voltage control such as induction regulators, in many cases. Transformers of special design are manufactured for such purposes as insulation testing, phase-angle control, and for supplies to electrical instruments. A special arrangement of windings, as in the Scott transformer for changing from three-phase to two-phase, may be employed for phase transformation. The protection of transformers against excessive damage in the event of a fault occurring is a matter of considerable importance, particularly in the case of the larger sizes where repairs of any magnitude necessitate the return of a unit to the manufacturer and are very expensive. One protective method, the Buchholz relay, depends on the fact that any fault, however slight, tends to form gases within the transformer. These gases are collected in a special receptacle in which a float device controls the trip coil circuit of the appropriate switchgear. The switch is tripped automatically before the fault has become sufficiently serious to cause extensive damage, and the windings can then be lifted from the transformer case for examination and repair. Such relays are sensitive and reliable, but are suitable only for units fitted with conservators. They are generally confined to large units" having vacuum treatment of the windings available before assembly at site, in order that the device shall not operate on account of the release of air bubbles trapped in the core and windings. Pro tection against line surges is dealt with in th~ next chapter. THE ROTARY CONVERTER AND MOTOR· GENERATOR For the conversion from alternating to direct current the most efficient rotating machine is the rotary converter (Fig. 16), which combines, in one frame, the equivalent of a synchronous motor and a direct current generator. If the alternating voltage is very unsteady Z9 and it is required to have a very steady continuous voltage, the motor generator is the better machine to use, and again, if it is desired to reduce the continuous voltage to zero and to bring it gradually to full value (as in the Ward-Leonard method of speed control), the motor generator is preferable. The motor-generator, being, as its name implies, two independent machines mechanically coupled, is also desirable in certain cases requiring change of voltage, as well as change from A.C. to D.C., where the use of a transformer with the rotary converter might be less economical. Formerly it was agreed that the motor-generator was more easily started after a shut-down, but as rotary converters are now designed to be self-starting and self synchronising this objection no longer holds. Rotary converters are widely used in traction and lighting sub stations. They are not self-starting from an alternating supply unless either specially wound so that they can be brought up to speed as induction motors or supplied with a starting motor on the same shaft. When a direct current supply is always available they may be started up easily as direct current motors. The D.C. voltage bears a definite relationship to the A.C. voltage although an automatic regulator combined with a synchronous A. C. booster machine on the same shaft may be used to correct variations which are not very sudden. Other methods of voltage variation are also available, within limits. Regula tion, compounding, and equalising are possible on the D.C. side by the provision of suitable windings while, on a three-wire network, the connection of the mid-wire to the star point of the low tension transformer winding in the supply to the converter makes the latter act as a balancer. The ,efficiency of a converter is highest when running at unity power factor, but it is often desirable to run at a leading power factor to compensate for other loads on a system. This is easily achieved by adjusting the field excitation provided that the converter has been designed to carry the extra current with out overheating. In 1902 La Cour and Bragstad introduced the motor converter, consisting of an induction motor and a p.C. machine coupled mechanically and with their rotors coupled electrically also. The machine acts partly as a converter and partly as a motor-generator, is self-starting, and may be used without transformers even when a high voltage step-down is required. Where change of voltage is required on a direct current system a motor-generator set must be used although both machines may be incorporated, in this case, in the same frame. If the full load efficiency of a rotary converter and transformer is taken as 94 per cent., that of a motor converter of the same rating will be about 92 per cent. and the motor-generator 90 per cent. The disparity between the relative efficiencies becomes very much greater at low loads. 30 STATIC RECTIFIERS The most widely used static rectifier for changing from alternating to direct currents is the mercury arc rectifier, the earliest form of which, the Cooper-Hewitt rectifier, was introduced in 1902. When an arc is struck in an atmosphere of mercury vapour current can pass only from a hot cathode to the cooler anode. Thus a unidirectional current is obtained from an alternating source and the steadiness of the direct current obtained increases with the number of phases in the applied A.C. supply. Originally these rectifiers were of small capacity operating in an exhausted glass bulb, but, in 1910, an iron case rectifier rated at I so amperes at 220 volts was built by Brown, Boveri, at Mannheim. Glass bulbs may still be employed up to about ISO kW. rating or even more, but the larger rectifiers built to-day are generally metal cased, and their reliability is now quite equal to that of the rotary converter. They are more efficient at low loads and are unaffected by frequency disturbances, but may give a "rougher" D.C. supply. The size of a rectifier depends on the ampere rating so that economic advantages are gained in designs for high voltages. They can be started up in a few seconds, auxiliary pumps being used to maintain the vacuum. Ratings between two and three thousand kilowatts are not uncommon. For currents greater than about soo amperes the question of cooling becomes difficult in the case of glass bulb rectifiers. It is claimed that the effect of a backfire in the rectifier, which may be caused through internal ionisation due to overheating of the anodes, is no more serious than a flash-over on a rotary converter if a circuit breaker functions quickly in such an eventuality. There is a definite ratio between the input A.C. and the output D.C. voltage so that an auxiliary transformer is normally required. This transformer may be arranged to provide, say, a six-phase supply to the rectifier from three-phase mains by connecting the secondary windings in 11 triple star " formation. The six-phase supply to the rectifier ensures a smoother D.C. output. Transformers giving a larger number of phases are also used. A common expedient in an iron-cased rectifier is to provide mercury seals at all joints which are also packed with asbestos. Clamped seals of heavily compressed material or moulded seals of micalex are also used. Tanks may be of steel, welded preferably by the atomic hydrogen process to eliminate porosity. The tightness of each, mercury seal may be indicated by means of wooden floats in the mercury feed-pipes. Porcelain or steatite insulators are employed wherever insulation is required, the complete rectifier also being insulated from earth. The use of special graphite tips and shields for the anodes prevents troubles due to melting, which were experienced with earlier metal types. Each anode is provided with a cooling radiator through which water may be forced, and the original striking of the arc is usually performed by means of a special ignition anode which is pulled up out of the cathode mercury pool. The arc pulled out in this way may be picked up by a set of auxiliary excitation anodes, 31 one to each pair of main anodes, fed from a separate auxiliary trans former. The use of these excitation anodes enables very small loads to be handled by the rectifier. Water-cooled anodes must be insulated from earth so that long lengths of rubber hosepipe may be used in the water-feed circuit and thermostatic water valve control may be employed to keep the rectifier temperature constant. The vacuum pumps may be arranged so that they are controlled from a vacuum gauge, such as the Pirani differential filament gauge, to keep the vacuum constant. In the Pirani gauge two filaments connected in the arms of a bridge circuit are run in the rectifier vacuum system and a sealed constant vacuum respectively. Any change in the rectifier vacuum alters the temperature at which its filament runs, thereby unbalancing the bridge. The McLeod compression gauge is unsuitable for automatic working but may be fitted as a more reliable check on the Pirani gauge. · Care must be taken to see that occluded gases are driven out of the components of the rectifier exposed to the vacuum before load is taken by a unit which has been opened up. One way of dealing with this contingency is to supply a small transformer and loading resistance which enables the rectifier to be given a preliminary run at low voltage and progressively increasing current. It may be considered desirable also to provide smoothing equipment such as a D.C. reactor and resonant shunts on the D.C. side in order to smooth the output and prevent interference with telephone circuits. Protection against arc backs may be obtained by installing high-speed reverse current circuit breakers on the D.C. side. The efficiency of a r,soo kW. B.T.H. metal-cased rectifier, illustrated in Fig. r8, was about 96·2 per cent. from half to full load, and 95'9 per cent. at 25 per cent. overload. This rectifier was put into commission in June I9JI. Its no-load loss is u·4 kW., power factor about 94'2 per cent., regulation about 4! per cent. D.C. voltage drop from no-load to full-load, and water consump tion about 4t gallons per day per roo ampere load. The guaranteed overloads were 25 per cent. for 2 hours, roo per cent. for ro minutes, and 200 per cent. momentarily. The efficiency of a rectifier becomes greater at high voltages, but they are unsuitable for regeneration as the mercury pool is the only electrode which can act as a cathode. Three electrode grid-controlled mercury arc rectifier-relays are now in the process of development in many countries, the G.E.C. of America having produced the" Thyratron" which, by the end of 1931, · had been made of the glass bulb type in sizes up to I 50 kW. and in which pressures up to 3o,ooo volts had been handled satisfactorily by 1932. There seems to be no insuperable difficulty to prevent pressures comparable with the highest alternating current transmission voltages being reached and in units of high-power• ratings being developed. Success in this way would give a great impetus to high· voltage direct current transmission. Some of the characteristics of grid-controlled rectifiers are extremely useful. Thus, the application of a momentary positive potential to all the grids in turn allows of J:Z voltage regulation of the rectified currents by controlling the phase incidence of the applied grid potentials. The application of a negative potential to all the grids stops the arc within one cycle of primary frequency, thereby enabling the rectifier to act as its own circuit breaker. The control grid reduces the possibility of an arc-back, but one of the greatest possibilities of grid control lies in the fact that such rectifiers may be so connected that either alternating current may be rectified or D.C. may be inverted into A.C. Certain other types of rectifier have been introduced. The "jet-wave rectifier " depends on the electro-magnetic production of a wave of mercury by the interaction of a constant magnetic field and the alternating current passing through a mercury jet. The wave is, in effect, a comrnutating device. Mention should also be made of the demountable thermionic valve, introduced in a form far superior to the earlier Holweck type, by Metropolitan-Vickers in 1931, in which year they produced a radio valve rated at soo kw. The main features of this design are in the grease seal between machined surfaces and the use of an oil vacuum pump which was the outcome of much research. The copper-oxide rectifier, introduced by Grondahl in 1927, has been developed commercially on a considerable scale, particularly for small supplies for instruments, radio sets, and small to medium sized industrial loads. A thin oxide film on a copper plate has been found to offer a very high resistance to the passage of current in one direction and a very low resistance in the other, or oxide-metal direction. Rapid development in the design of static rectifiers for all purposes seems to point to the possible exclusion of the rotary machine for voltages above 6oo. As an example of the flexibility now achieved a mercury arc unit built by Siemens-Schuckert in 1932 may be quoted. It converted three-phase so cycle supply to single-phase 16~ cycle supply at rs,ooo volts and had a capacity of 2,500 kW. Before leaving the subject of static rectifiers it is appropriate to mention that the researches of J. J. Thomson, from r888 onwards, into electric conduction through gases, led to the establishment of an electron theory on which a proper understanding of the working of the mercury arc and the thermionic valve depends. . FREQUENCY CONVERTER In addition to the recent application of mercury rectifiers to frequency conversion, rotary machines have been widely constructed for this purpose. They consist of an armature similar to that of a rotary converter, and an unwound iron stator. Three brushes, spaced at 120°, are placed around the commutator and the incoming A.C. is fed through the slip rings. From the three brushes a three phase supply can be taken, the frequency of which depends on the speed at which the annature is driven. . 33 HARMONIC ABSORBERS The wave form of the voltage of a large network with a number of high capacity transformers permanently connected is likely to become distorted to an appreciable extent at light loads by the unabsorbed fifth harmonic introduced by the magnetising current. The more serious third harmonic is usually absorbed in the delta, or tertiary windings. Harmonic absorbers, to correct the fifth harmonic distor tion, may consist either of a three~phase reasonant circuit consisting of static condensers and a transformer, with automatic regulating resistance and control gear, or of a rotary machine on the low voltage side of the system. The rotary machine acts as a synchronous motor with windings so arranged that the quintruple frequency current is drawn from the system and, by rotation of the stator, at the correct phase relative to the fundamental. Adjustment, in each case, is made with the aid of a voltmeter tuned to respond only to the harmonic frequency. SWITCHGEAR AND PROTECTIVE DEVICES One of the most important requirements of a supply system ' is maximum continuity of service, and for this reason standby apparatus is always installed for use in the event of a breakdown or a stoppage for overhaul and repairs. In addition, a system is usually arranged so that there are alternative means of bringing a supply to strategic points so that, in the event of a complete breakdown at any portion, the effects can be localised. Means are also employed whereby apparatus is automatically disconnected from a circuit before overload conditions due to abnormal demand from consumers, transient disturbances in a line, or breakdown of insulation become so great that extensive damage might ensue. · The operation of making, or breaking, a circuit is accomplished by the insertion or withdrawal of some sort of conducting link insulated from earth. If the operation is always to be performed when no current is flowing in the circuit the link takes the form of a simple knife switch, manually operated. If the circuit must be broken when current is flowing a " circuit breaker " or isolating switch specially designed to clear without damage the power arc consequent on rupture is usually employed. An isolating switch (which may be an ordinary knife switch with either a spring mounted section to the blade, or with arcing horns, designed so that the current is not broken at those surfaces required for contact when the switch is closed) is manually operated and is intended for breaking only the normal line current. A circuit breaker may be required to rupture, not only the ordinary line current, but the enormous surge which may follow a short circuit on the line, and its action must necessarily be automatic under the latter condition. The magnitude of such a surge depends upon the lowness of the impedance of the circuit between the generator and the point at which the short circuit occurs. As the impedance .increases progressively as transmission lines and intermediate apparatus increase in length and quantity respectively, it will be appreciated that circuit breakers immediately controlling a generator must be of high capacity, while those remote from the generator may have a continuously diminishing capacity. The mechanism through which a circuit breaker is closed may be• so arranged that it may be tripped and the circuit broken through the agency of coils actuating an electromagnetic tripping device. These coils may operate directly (or through instrument transformers or shunts connected to the line circuits) on overloads, leakage current, or no~volts, or may be energised by the closing of relays in the trip coil circuit. Circuit breakers may be opened or closed either electri· cally or by hand. Electric operation may be by means of a small remotely situated switch controlling the circuit of a motor or solenoid device mounted with the closing mechanism on the breaker. Where 35 hand operation is desired it is usual to employ a " free handle ,. device. That is, the handle is so connected to the mechanism that it is not violently actuated if the breaker trips while the operator has it in his grasp. The earliest circuit breakers were of the " air break " type, and these are still used extensively in improved form for low voltage circuits up to thousands of amperes capacity. The air~break knife switch, which produced a very long arc even at low pressures, was practically the only device used prior to 1897· Modem air circuit breakers are mounted on slate or composition panels and usually break the circuit with great rapidity by means of powerful springs which are extended or compressed by the closing operation. The main contacts are of copper, specially designed to give an adequate contact surface, and carbon auxiliary contacts are usually provided, so arranged that they remain in circuit a little longer than the copper and therefore take the full effect of the arc. Such contacts are easily replaceable if they become seriously burnt, and they are not depended upon for current carrying except at the moment of breaking. Arc shields of asbestos compound are often placed between the poles of such a breaker and a magnetic blow-out coil may be introduced to assist in the rapid dissipa~ tion of the arc. The main object in these breakers is to achieve their purpose with great rapidity and therefore, by reducing the time during which the arc may be maintained, to lessen the possibility of injurious heating. For higher voltage working it is necessary to employ other means to ensure that the power arc consequent upon breaking a circuit under load is not sustained. The most common device adopted at present is to immerse the breaker contacts in oil, C. E. L. Brown of Brown, Boveri, having used this method in I 897. The oil cools and com presses the arc, and, on alternating current circuits, tends to extinguish it at the moment when the current becomes zero. The factors govern~ ing the performance of an oil circuit breaker are, however, very complex, and a few: may be enumerated. I. Considerable energy is liberated within the breaker, and this results in heating, vapourisation, and breaking up of some of the oil. The hot bubble of gas formed provides a medium well suited to the maintenance of an arc but, at the same time, the bubble promotes a violent turbulence in the oil causing a fortuitous projection of cold oil into the arc. 2. The arc is most easily extinguished at the moment when the current is zero, and the earliest possible zero is desirable as each succes sive half wave of current liberates more energy. De-ionisation of the arc path should therefore ensue as soon as possible, but, if the voltage is lagging or leading the current a potential exists across the arc path at the instant when zero current is reached. The worst case of this is at zero power factor when the voltage is a maximum when the current is zero. Extinction thus becomes a struggle between building up J6 FlG. I9.-0il Circuit Breaker, soo amps., I I ,ooo volts, 1903. [Dcsig11ed by If, W. Clothier and B . Price. FIG. 20.-0riginal Reyrollc Metal-clad Switch Unit, 1905. To /au P«t< 37.' [Reyrolle & Co ., Ltd. FIG . 21.-Horizontal Draw-out Metal-clad Switch Unit. Duplicate Bus·blr, solenoid operated, 2,ooo amp., 6,6oo volt, ] · phase oil-immersed circuit breaker. [Rzyrolle & Co., Ltd. FIG. 22.-Control Board at Barking Power Station. Yiew sbo";ng generator section of control board ·with go\·emor motor and field pedestals and d esk panel. for operatjoo of station auxiliary board . dielectric strength in the arc space and the building up of the voltage across it. 3· The energy present in a circuit breaker during interruption appears to be a direct function of the ohmic resistance of the arc. A short arc therefore means that less energy is liberated, but speed of break is an important factor in obtaining conditions suitable for de ionisation. The more speedy the break the longer the arc is drawn out during each half cycle of current and the greater the energy liberated in a given time. The object, therefore, in all oil circuit breakers is so to combine the numerous opposing factors that the best performance is obtained. Design, partly on account of the enormous expense of conducting tests at large breaking capacities, has been largely empirical and ratings, in many cases, are somewhat arbitrary. This has applied particularly to large breakers where a short circuit rupturing capacity of I,5oo,ooo kVA. or more may be required by calculation of circuit conditions. Latterly much more attention has been paid to really scientific research into switchgear problems. General points in design are :- I. In a multi-break contact there is more chance of one of the breaks being early effective. This point seems first to have been appreciated by Ferranti. 2. The tank must be strong enough to resist the forces consequent on interruption, and there should be an outlet for the liberated gases. 3· There is an optimum value for the speed of break. In the ordinary oil circuit breaker no special means, other than baffles to break up the oil, are taken to control the arc. Such breakers are quite satisfactory for the more usual rupturing capacities and voltages. In other types baffles have been fitted with the idea of directing cold oil on to the arc or magnetic blow-outs have been incor porated in order to blow the arc into the cold oil. In the case of the " explosion pot " breaker the fixed contact is enclosed in a confined space made of insulating material through the throat of which the moving contact enters and is withdrawn when interruption occurs. In the process of withdrawal an arc is drawn out in the enclosed space, generating gas under pressure. The only outlet for this gas is through the throat, and the arc must also pass through this outlet when the moving contact is withdrawn. The result is a violent turbulence as the contact leaves the throat of the explosion pot and the arc is extinguished. A number of designs have been introduced of high-tension circuit breakers which use means other than oil filling for quenching the arc. Among these are the u de-ion " breaker introduced by the General Electric Co. in America, in which the arc is broken up into a large number of small sections and is then magnetically extinguished. An oil immersed " de-ion " type breaker has also been developed. The u expansion '' breaker, a development of a very early type introduced 37 by Raworth for the Ban.kside Power Station in London, has been con structed during 1931 in large sizes in Germany. It depends for its arc breaking properties on the adiabatic expansion of steam liberated by the heat from the arc. Gas blast, or air blast breakers, first intro duced in America in 1901, have also been the subject of recent attention and great improvement. The withdrawal of the contact opens a gas valve, the outlet for the gas being through the fixed contact and along the arc path. The onrush ot a large quantity of de-ionised gas under pressure extinguishes the arc. These breakers may be operated pneumatically, being so arranged that they cannot be closed unless there is a sufficient head of gas or air in the storage chamber. There are also oil blast breakers, the principle having first been used by Ferranti early in the present century. A jet of oil is forced under pressure through the arc, the pressure of the liberated gases within the breaker being employed for this purpose. Recent developments of the oil blast principles have given very satisfactory results on test, several patents having been taken out, including those by E. B. Wedmore and W. B. Whitney of the Electrical Research Association in I9JI. De-ionisation of the arc path by the oil jet has been achieved with very small separations between contacts compared with the straightforward type of oil breaker, with the added advantage of an increase in the rupturing capacity. There is still, however, a fire risk with oil. Circuit breaker contacts are usually carried through porcelain or bakelite insulators, the latter being if necessary of the condenser type, and the contact arm carriers may be of hard wood or bakelite. The insulators on outdoor breakers must be weatherproof, and porcelain, steatite, or in some cases stoneware rain-shields are employed. Arcing contacts and current limiting resistances may be fitted to the breaking arms. Circuit breakers, isolating links, and busbars (the main supply conductors from which branch supplies are tapped) must be housed in such a manner that danger to persons having access to their vicinity is obviated and fire risk is reduced. In the earliest days of power supply, switches, fuses, and meters were mounted on wooden switch boards and were largely unprotected (e.g. Edison, 1882) until the danger was appreciated and boards such as the Siemens and Lowrie Hall types were introduced. For low voltage schemes where trained operatives only have access to the switchboard, a similar scheme is still employed except that the wood is replaced by slate, marble, or an ebonite asbestos compound. All instruments, meters, relays, fuses, switches, circuit-breaker handles, and indicating devices required for operation are mounted on the front of the board, and oil circuit breakers, isolating switches, busbars, cable boxes, shunts, or current transformers, and small wiring which normally require no attention are mounted at the rear and are often screened. For higher voltages other forms of construction are used in which, if the operator can obtain access to an energised connection at all, it is only with deliberate intention or gross carelessness. J8 Ferranti, in 1888, installed switchgear at Deptford for 10,000 volts working. The high tension connections and switches were mounted on porcelain or ebonite insulators attached to the surface of a wall, with the operating gear mounted below. In 1894 he improved on this arrangement by introducing the open cellular type of construction in which the high tension portions were protected by slabs of insulating material. Other means of protecting the " live " connections included the " pillar " type of switchgear introduced by Raworth and the " basement type " favoured in Germany, but the only type of construc tion which has survived to any extent is the " cubicle " switchboard in which all energised apparatus is housed within the compartments of a sheet-iron cubicle or a moulded stone cubicle with sheet-iron doors. Groups of apparatus such as bus bars, isolating links and circuit breakers may be kept in separate sections of the cubicle and a whole board may be built up of several cubicles along the length of which run common busbars. The doors may be fitted with simple stops, or interlocks, so arranged that unless the circuit breaker is in the " off " position the door of the breaker compartment cannot be opened and none of the other doors can be opened unless the " breaker '' door is first opened. In the " truck " cubicle, which superseded the " hinged panel " construction of Cowan, and which was introduced in this country as early as 1903, all the apparatus except busbars and cable boxes, and possibly instrument transformers, is mounted on a movable truck with plug contacts. Sockets are connected to the busbars and the cable box terminals. The withdrawal of a truck is prevented by interlocks unless the circuit breaker is in the " off " position and a shield drops over the busbar socket holes as the truck is withdrawn. Only the operating handles are accessible unless the truck is withdrawn from its " house," in which case the apparatus made accessible is "dead." The "house" is a sheet-iron cubicle into which the truck fits. Metal-clad units are a development of the truck cubicle and may be built economically up to very large ratings (e.g. a switch of 2,ooo,ooo kVA. rupturing capacity or more). The earliest metal-clad unit was built by Reyrolle & Co. in 1905 • (Fig. 20). Normally only the circuit breaker is movable and the insulators of the breaker are equipped with plugs which enter insulated sockets connected to the busbars. If duplicate busbars are required there may be two positions from which the breaker can be inserted into appropriate sockets or a bus-selector switch may be employed. The weight of the circuit breaker often necessitates motor operation for insertion, and the resistance of the mechanism may require solenoid operation for closing in the case of a large breaker (Fig. 21). Metal-clad units may be of either the vertical or horizontal drawout type. The stationary, or " house " portion of the unit is divided into the usual compartments, but these are filled with oil or compound to economise space over that which would be required for air insulation. It is usually unnecessary to have isolating switches in either a truck or a metal-clad unit as the with drawal of the circuit breaker from the busbars accomplishes the same 39 purpose. Owing to the closer spacing of the conductors in metal-clad units the busbars must be more rigidly fixed to resist short circuit forces. As in the truck cubicle a shield drops over the energised busbar socket holes as the circuit breaker is withdra\\-11, Cubicles and trucks are not usually employed for voltages above 33,000 owing to the great clearances which would be required in air. The main applicability of these types seems to be from 2,000 to about n,ooo volts. Metal-clad units are used mainly up to 33,000 volts, but they are also constructed economically up to about 66,000 volts and units for IJ2,000 and even higher voltages are now being developed. For industrial purposes, such as in mines and mills, where the available space may be very limited, small metal-clad units have been developed. The exclusion of vermin, which are often the cause of short circuits, is a great advantage of these types of construction. For very high voltages it is most economical to construct outdoor substations as building costs, owing to the high clearances necessary, would be prohibitive, while even where voltage is not the determining factor it is also economical in many cases to employ the outdoor type of construction. It is essential that all apparatus should be completely weatherproof and that the substation should be enclosed efficiently against unautho rised entry. All live parts must be supported at a reasonable distance above ground so that adequate clearance is obtai~ed from an operator walking underneath. The supports may take the form of reinforced concrete pedestals or a steel frame, and the general layout of asubstation is subject to considerable variation according to general requirements. Less ground space is required by the " high frame " construction (Fig. 24), and if the frame is of steel it is claimed that protection from the effects of lightning is thereby obtained. Where ground is cheap, however, the "low frame" construction (Fig. 23} may be more economical, and it is certainly simpler and the apparatus is more accessible. A type of outdoor substation employing oil-filled cables as busbars, and oil-immersed isolators, has been designed for restricted sites and high voltages and was first employed on the State Line Power Station installation in America. For small branch tappings from a transmission line where the cost of a building would be prohibitive, pole mounted transformers and air break isolating switches or fuses may 'be used, or the substation apparatus may be mounted in a weatherproof sheet steel kiosk which must be ventilated to prevent u sweating " due to the rapid changes in outside temperature to which it may be subjected. Substations may be attended by operators or may be controlled from a distant position by means of a remote control board, in which case only periodic inspection is required and the control of a number of widely-spaced groups of apparatus can be centralised. A remote control board is usually fitted with instruments indicating load condi tions and indicators from which circuit conditions can be seen. The 40 latter indicate whether circuit breakers, isolating links, and bus-con nectors are " on " or " off " by means of pilot lamps or electrically operated semaphores. Distant operation of apparatus is usually obtained through light current circuits which close relays in a local heavier current supply to the operating gear. The connecting circuits are usually multi-core cables of fairly light gauge. The indications required on any feeder or group of feeders at any particular switchboard position may be none, or any combination of the following :-volts, amps, watts, power factor, frequency, watt-hours, maximum demand, leakage current, and, in special cases, phase rotation and synchronising indications. In a three-phase balanced system one phase may be taken as typical of the others for the first three measure ments, but on unbalanced systems each may be required separately. The trip coils on the switch may be relied upon entirely for protection or relays may be required. Relays may be arranged to operate on overloads, earth faults, over or under voltage, or reverse currents. A time switch or voltage regulator may also be necessary. The accommodation of a large number of instruments on a panel, which may already contain the circuit breaker lever gear, is often a difficult problem, particularly in truck and metal-clad units where space is' limited by standardised design. In addition the volt-ampere rating of current and potential transformers must be chosen with regard to the instrument and trip coil load they will have to bear. The action of relays depends in most cases on the closing of a trip coil circuit by the bridging of a pair of contacts due to an electro magnetic or inductive movement within the relay. They are usually provided with means whereby they may be set within certain definite limits and may be designed to have a damped characteristic so that they do not operate unnecessarily on a momentary fluctuation in circuit conditions. In the case of ordinary trip coil protection such a character istic can be obtained by short circuiting the coil with a fuse which will not blow until a certain time has elapsed depending on the current flowing through it. The trip coil cannot act until the u time limit fuse " has blown. In the case of relays those which are not " instantaneous " usually have an" inverse time limit "characteristic whereby the_amount of delay in operation is inversely proportional to the magnitude of the actuating current. Reverse current relays require to be compounded if they are to be practically independent of the supply pressure. Relays may be combined with special circuits in order to provide " balanced protection " against faults on the principle that, under normal conditions, the currents flowing in the main circuits have a definite relationship to one another. Such differential protection may consist of: {1} End balance protection in which the current entering a piece of apparatus is compared with that emerging from it. (2) Parallel balance protection in which two or more exactly similar paths are provided for the current, which divides equally when all is in order. (3) Phase balance protection in which the currents in, say a three phase system, are added together, the sum amounting to zero when there is no fault. The development of photo-electric cells in recent years, together with the gas-filled relay and the evacuated valve, has led to the intro duction of a number of new applications which may replace electro magnetically operated devices in certain cases in the near future. They may be combined to act as relays, meters, controls, inverters, amplifiers, converters, and switches in cases where their characteristics are suitable to the performance required. Protective devices which are independent of switch operation include choke coils or reactors designed to reflect transitory surges away from apparatus such as generators or transformers. They operate by virtue of a high inductance, which offers a high impedance and time lag to any high frequency surge. If a choke coil is placed at the end of an overhead transmission line it will in most cases tend to reflect a surge due to, say, lightning, back along the line where its effects may be cleared by a flash-over or leakage across one or more insulator strings before any serious stress is experienced by the substation trans formers. Lightning arrestors, which may be electrolytic or of the spark gap type, may also be installed. They offer a very high resistance path to ordinary line voltages, but allow the passage of a heavy current when their breakdown voltage has been reached. The disadvantages of these arrestors lies in the fact that the critical voltage at which they operate is often hundreds per cent. above normal line voltage. In a hydro-electric scheme, for insta,nce, where voltage variation may be comparatively large, due to loss of load or overspeed, the setting of the usual type of arrestor must be relatively high in order that it shall not operate on ordinary power frequency fluctuations. The " surge absorber" has been introduced to overcome these disadvantages. It consists of an air core inductance surrounded by an earthed metallic sheet. The inductance coil acts as the primary of a transformer, energy being dissipated in the secondary consisting of the one short circuited tum. In addition, the magnetic field of the inductance sets up eddy current losses in the shield,. and the combination of the inductance and the electrostatic capacity to the shield forms a high frequency filter circuit which tends to flatten the u ripples " in the voltage curve due to a surge. The surge absorber, by virtue of this threefold operation, is claimed to be effective in dissipating any sudden rise in voltage. The provision of weak links at strategic points in the insulation of a system is, however, one of the simplest and most common methods of protecting apparatus connected to overhead lines. Fuses, which have been mentioned, but not described above, are in effect circuit breakers which are not immediately reclosable. They usually consist of lead, lead alloy, or tinned copper wires or strips which will fuse and break the circuit if subjected to a serious overload. Some of the earliest attempts at protection of electrical systems and apparatus were based on the employment of fuses, designs having been 4Z made by Sylvanus Thompson, Lord Kelvin, Edison, Hedges, and others. Modem fuses are totally enclosed in a porcelain, fibre, glass, or composition carrier. In some types the wire can be replaced easily, but in cartridge types, where the cartridge is commonly filled with a non-combustible powder, it is convenient to refit a new cartridge after the fuse has blown and to rewire the old one for another occasion. Fuses are most extensively used on low-voltage low-current schemes, as in supplies to consumers. Quick break cartridge fuses have been developed for voltages up to JJ,ooo, in which the wire is enclosed in a glass tube under the tension of a spring which contracts as soon as the wire fuses. Heavy current fuses are used mainly on low-tension switchboards. 43 TRANSMISSION AND DISTRIBUTION In the early days of electric supply, when machines were of .small output and consumers were concentrated within a small radius, the only type of distribution required was that which to-day is usually the final link in a complicated system, namely, the low-pressure supply to the consumer. It is now necessary to transmit power over distances varying up to hundreds of miles in order to obtain the economies consequent upon large scale generation at favourable sites, and in order to effect interlinkage between supply systems which reduces the seriousness of a breakdown and makes possible the more economic apportionment of loading. It may be stated, as a generalisation, that the longer the distance over which it is desired to transmit power the higher is the voltage at which it is most economical to transmit. The raising of the trans mission voltage increases the cost of insulation, switchgear, and trans formers, but, against these factors, the price of the conductors is reduced. For a given resistance loss the conductor area, and con sequently the weight per mile of line, varies inversely as the square of the voltage (or directly as the square of the current). Reactance and capacity losses, and leakage, have also to be taken into account. The procedure in determining the best transmission voltage for any particular scheme is therefore to estimate the cost of a line at various voltages, including, as well as the above items, the cost of supports, wayleaves, and erection. It will be found that for any given set of conditions the curve of costs plotted against voltage will reach a minimum value and a standard voltage near this minimum may be adopted for transmission. The problem may be complicated by the necessity for estimating load developments expected in the future, and a decision must be made as to whether overhead line or underground cable construction will be adopted and whether, in the former case, an earth wire will be included to help in safeguarding the line against lightning surges. Cables are used mainly in populous districts and overhead lines mainly in the country. The chief difficulties in the way of overhead line develop ment in towns are the overcoming of strong local prejudice and the provision of tower sites. For long distance main lines a voltage of uo,ooo-220,000 volts is usual, and even higher working voltages, such as 38o,ooo in one very extensive German scheme, are employed. · For branch lines and shorter distance mains voltages from 22,ooo- 66,ooo are most common. For secondary branch lines and still shorter mains rs,ooo volts down to 2,200 volts may be employed. Distribution lines are usual1y at I ro-soo volts. 44 [To face page 44. Photo: G. Marshall Smith, London.] [Central Electricity Boaro FIG. 2J.-IJ:Z,ooo-volt" Grid" Transforming Station. Low Type. [Afetropolitan- Vickers Electrical Co., Ltd. FIG. 24--IJ:z,ooo-volt" Grid" Transforming Station. High Type. To faa page 45.] Photo : A . Winta , Preston.] [Cmtral Elutridty Board. FIG. 2S.- IJ2,ooo-volt" Grid" System. River Crossing at Preston. Nearly all main transmission lines are three-phase, although single phase or two-phase working is sometimes employed. High-tension direct current transmission has numerous advantages to commend it, but, apart from the little used Thury system in which all connected apparatus had to be insulated for full line voltage, the advent of suitable means of transformation and control has been awaited. The use of direct current increases enormously the carrying capacity of long lines in which inductive and capacity drop are elimi nated, and for which the power-factor of the generator load is very nearly unity. The grid-controlled mercury arc rectifier may stimulate progress in high-tension direct current transmission as, using such apparatus, alternating current can be generated in the usual manner, stepped up to a suitable voltage for transformation into direct current by a rectifier, then transmitted, after which an inverted rectifier can again produce alternating currents for distribution. By such a scheme the replacement of existing apparatus is minimised, expensive synchronising and regulating apparatus is unnecessary, the system is exceptionally stable, and the grid-controlled rectifier can act as its own circuit breaker. The application of a negative voltage to all the grids in the case of a fault occurring stops the arc in less than I cycle of primary frequency. Distribution may be either direct current, usually through the medium of converter substations, or single-, two-, or three-phase alternating current. The first distribution scheme of any magnitude apart from individual arc lighting schemes was that employed by Edison in New York in r 881. Underground conductors were laid in iron tubes IO feet in length, each tube containing two conductors of semi-circular cross-section insulated from each other by spacing pieces and surrounded by asphalt or bitumen. The pressure was I 10 volts. In r882-3 Dr. John Hopkinson and Edison independently invented the three-wire system of distribution which is almost universally employed for direct currents to-day. The generators develop double the supply voltage required for two-wire distribution, and the load is connected between either line wire and a neutral wire. The neutral has to carry a current due only to the difference in the load on each half of the system and need be only of relatively small section. In addition, as each line wire carries only half the current which it would carry on a two-wire system, it need be only a quarter as large in cross-section for the same losses. The result is that the three-wire system entails a saving of about 70 per cent. in conductor weight over an equivalent two-wire system. AB inequality in the load on each half of a three-wire system would cause uneven division of the total voltage between the outers and the neutral, balancers are now introduced. In addition, as it is necessary to correct for voltage drop through resistance losses along the line, voltage boosters are inserted at intervals or feeder lines are run direct to distant points. In this country the Board of Trade Regulations specify a maximum permissible voltage variation of 4 per cent. at any 45 point on a system of low tension distribution (I2l per cent. on e.h.t. lines). On alternating current systems, for transmitting a given amount of kilovolt-ampere load and allowing the same permissible losses in each case, the various systems would require the following relative weights of conductor : Single-phase two wire or two-phase four wire . , 1oo Two-phase three wire or three-phase mesh connected 75 Three-phase star connected 27 The power factor of the load on an alternating system may reduce con siderably the effective capacity of the conductors, unity power factor being the ideal condition. For this reason many supply companies encourage, by preferential rates, the installation of apparatus for power factor correction on consumer's premises and may install such apparatus in their own substations as well. The additional cost of condensers or machines running at a leading power factor may be more than offset by the reduced expenditure on generating plant, switchgear, and mains. Low tension distribution, in cities, is usually by cables carried underground in ducts. Main distributor cables run from a substation to a pillar unit or underground link disconnecting box where they are connected through removable links or fuses to a number of outgoing secondary distributor cables which radiate through a consuming area. Where these outgoing distributors pass consumer's premises a service box is installed. At these boxes service cables tee off to the consumer without disturbing the continuity of the distributor which serves many consumers. The service cable is terminated at the consumer's premises in a fusible cutout. The consumer then takes his supply through a meter and his own fuses to his wiring system. Faults in a consumer's system are confined to his own wiring by his fuses, which, however, may have been inexpertly wired. In such a case the company's service cutout functions. A faulty secondary distributor is sectionalised by a fuse unit in the pillar or distribution box so that the main distributor still continues to feed all the secondary distributors except that directly effected by the fault. The main distributors are sectionalised at the substation. Continuity of supply is thus ensured to all sections of a distribution network except a closely localised portion in the event of a fault developing. Distribution lines may also be carried on poles or on brackets fixed to the outer walls of buildings, in which case, although the design of the fittings is altered accordingly, the principle of sectionalisation is similarly adopted. High voltage transmission, using transformers in series, was suggested by Jablochkoff in 1877, and was improved upon by Gaulard and Gibbs in 1882. The work of Zipemowsky, Deri, and Blathy in 1885, and of Stanley in 1886, in introducing a system of transformers in parallel represented a great advance. Ferranti employed a system on 46 these lines for the old Grosvenor Gallery Station in London, and in I 889 he was responsible for the establishment of the Deptford Power Station of the London Electric Supply Corporation supplying single~ phase current at a pressure of zo,ooo volts to transformers in the London area over a distance of 7 miles. The current was at first carried, by rubber-insulated overhead cable,• and later by underground cables • consisting of two concentric copper tubes insulated from each other and from an iron casing by manilla paper impregnated with resin (Fig. 26). These cables could be manufactured in lengths of only 20 feet, and the lengths had to be jointed together. The use of paper for insulation was a revolutionary step, the importance of which can be gauged from the fact that to-day paper is still the most widely used insulating material for underground cables, while in those days rubber, which was used in L.T. cables, cost as much as 2s. 6d. per lb. In 1902 a so,ooo volt underground cable was manufactured for the Dusseldorf exhibition, and paper insulated cables are now in common use up to 66,ooo volts. The conductors are stranded and the sheaths are made of lead with, possibly, steel tape or wire armouring, in order that the cables may be sufficiently flexible to be made in long lengths and then coiled for transport. The joint is usually the weakest spot in a high voltage cable system, and therefore as few as possible are included in a run of cable. Cables for high voltages were found susceptible to damage by creepage of the insulated core under the lead sheath leading to the formation of small air pockets between insulation and lead, in which corona discharge was set up. This trouble has been largely overcome by the introduction of the " H " type cable in which three conductors are separately insulated and then shielded by cementing a layer of metallised paper to the outside of each core. The three cores are then laid up together under one lead sheath, and the whole may be steel tape armoured. In the" S.L." type three single lead-sheathed cables are laid up together. The u H.S.L." type is a modification of the "S.L." type in which damage by creeping of the core is eliminated as in the" H" type. The metallised design is due to Hockstadter. The advantage of the three types of cable described above, over the earlier unshielded types, is that the electric stress is purely radial and filling material between cores, in which minute air pockets might be formed, is excluded from the electric field. Such cables are manu factured for voltages up to 66,ooo, the" H "type being very extensively used, although for certain climatic conditions, on steep gradients and" under water, the" H.S.L." type is preferable. The latter type is also easier to joint but is more expensive. Multi- or single-core cables, in which the dielectric is oil impregnated, were introduced for extra high voltage work by Emanueli, of Milan. the first commercial 132 kV. production being in about the year 1925. The oil level is maintained by special reservoirs, and such cables are in use on several IJZ,ooo volt systems and are being tried on zoo,ooo volt circuits. They require careful laying and considerable attention, but 47 it is claimed that they are satisfactory in service. They may be run at much higher temperatures and with a steeper electric gradient across the insulation than ordinary paper-insulated cables. Cable joints are made at special joint boxes, the insulation of the two ends, in the case of paper-insulated cables, being stripped back gradually after which the conductors are bonded together. Each joint is then re-insulated by hand and the box is filled with insulating compound. The process requires considerable skill if a satisfactory joint is to be made. Rubber insulation is now mainly confined to low-voltage small wiring or trailer cables. " Cab Tyre Sheathed " cable, introduced by the St. Helens Cable Co. in I9Io,• and having a very tough coating of hard vulcanised rubber, is often used in arduous industrial applications. Parallel with the development of underground cables for trans mission the overhead line has made great progress. A direct current overhead line at 2,000 volts was constructed by Deprez between Miesbach and Munich in z882, and in 1891 a three-phase overhead line was constructed by Oscar von Miller between Lauffen and Frank fort, in Germany, carrying a supply at zs,ooo volts over a distance of 178 kilometres. The conductors were supported on porcelain oil insulators. The early pin type supports and shackle type tension insulators were severely restricted as to both their electrical and mechanical characteristics. In 1906 Hewlett introduced the first suspension type of insulator consisting of a porcelain unit which could be joined flexibly to any number of similar units in order to build up a string having the desired electrical characteristics. The line was suspended from such strings instead of being attached to the head of a rigid pin insulator. The development of the suspension insulator has made possible the construction of lines at working voltages up to 38o,ooo volts and very high mechanical loadings are now possible. This latter feature is particularly evident in the case of long river or valley crossings where the use of special high strength conductors under a strain of many tons enables the line designer to employ spans of several thousand feet and thereby to avoid a very long detour, or to carry a line to a district which could not otherwise be served economically. Overhead transmission lines are subject to enormous variations in design due, in addition to normal technical requirements and individual preferences, to climate, local supplies of material, fluctuating com modity values, and physical contours. The most familiar material for conductors in the early days of overhead distribution and telegraphy was galvanised iron wire, some times with a further protective coating of linseed oil or bitumen. Copper-covered steel was also used in America, first consisting of tinned steel wound with copper tape and then passed through a bath of solder and later, in 1877, consisting of copper electrically deposited on the steel. As an example of the range within which the designer may now work, conductors may be of steel, copper, cadmium-copper, 48 copper-clad steel, copper-cored steel, aluminium, aluminium-bronze, or steel-cored aluminium ; the lay up in the case of stranded conductors may be with plain circular strands or of strands specially shaped and pre-twisted to give a solid section, or the core may be left hollow. The more commonly used of the above conductors are copper and steel cored aluminium, and, for small power loads, steel. The steel core is introduced into steel-cored aluminium to improve the mechanical characteristics of the composite conductors. For the IJZ,ooo volt lines of the " Grid " scheme in this country a conductor is employed having a maximum permissible working load of 8,ooo lbs. and con sisting of seven steel wires o· I I inch diameter surrounded by thirty aluminium wires of similar area. In certain cases lines have been constructed of copper mainly on account of the better security for raising foreign loans which is offered by the high scrap value of copper relative to aluminium. One decided advantage of aluminium for extra high voltages, however, is that the larger area required makes the point at which corona discharge takes place higher than that of the equivalent copper line. The critical corona discharge voltage increases with the diameter of the conductor. Supports may be in the form of wood poles, tubular iron poles, expanded steel sections, reinforced concrete, composite tubular poles, such as the " Kay " pole, or fabricated lattice steel masts. Wood poles are used mainly for the lower voltage lines, up to JJ,OOO volts, but in America, for instance, a wood pole construction has been used up to I6s,ooo volts. It is common for a line to be constructed with wooden intermediate supports where the loading is due mainly to the tranverse wind loading on the wires (and on any ice which may form on the wires}, and with lattice terminal or angle supports where the much heavier loading due to the end or resultant tension in the conductors is experienced. Wood poles, combined in " H " and " A " or square formation and further supported by stay wires, may be used where more strength is required than could be given by a single pole. Tubular iron poles may be used instead of wood, particularly where it is feared that the creosoted butt of a wood pole will quickly rot or be destroyed by insects (as in parts of the tropics) or where the appearance is preferred. One great disadvantage of the tubular iron pole apart from cost is that the inside is difficult to inspect and paint while in service. For this reason, and also on account of price, expanded steel sections have been introduced. These sections are erected with the deep face transverse to the load and are relatively weak in the direction of the line pull. Theoretically an intermediate support does not experience any unbalanced line pull. Reinforced concrete poles are used extensively, mainly abroad. It is claimed that they require no maintenance and are practically indestructible. On the other hand, they are bulky and heavy to transport and may be liable t . seasonal cracking if design and manufacture is not well controllec ·. They are surprisingly flexible, and may be either centrifugally cast or shuttered and cored in the ordinary way. They are sometimes fabricated on site. 4-<261) 49 Lattice steel towers have the advantage of being almost unlimited in their application. They can be used up to the highest voltages and may be conveniently designed for almost any load. With reasonable maintenance their life is excellent, and economic reasons alone preclude their adoption for almost all lines. One of the earliest lines of import ance supported on lattice towers was built in 1905 for the Niagara, Lockport, and Ontario Power Co., to transmit power at 6o kV. over a distance of 200 miles. Some of the structures were built as tripods, others with tubular members, and the lines were supported on pin insulators arranged in triangular formation. With the introduction of the suspension type of insulator, tower design received closer attention and the permissible transmission voltage became rapidly higher. Two circuit lines (6 conductors) became more common and, with increased clearances for higher voltages, cross-arms were extended, making evident the necessity for designing for considerable torsion loads in the event of conductor breakage. Hinged cross-arms are now sometimes used to reduce the radius at which the torsion acts in such an eventuality. It was found that if conductors were ice laden the ice might suddenly fall off one conductor only, making it whip up and strike one of the others. The same thing might happen if a large number of birds perched on one line were suddenly disturbed. Con ductors were therefore staggered with regard to one another so that a short circuit from such causes became very improbable. Lattice towers up to between four and five hundred feet in height, for carrying long spans with adequate clearance over navigable waterways, have been constructed (Fig. 25), and it is now common for towers to be designed for a very large number of parallel circuits for positions where the paths of such circuits lie together. · Lattice towers may be transported economically in a knocked down condition and bolted together on site. They are usually painted with at least two coats of oxide or graphite paint, although in some cases they are completely galvanised, which is more expensive. It is common for the top section only, to a distance of a few feet below the bottom cross-arm, to be galvanised, and the rest painted. Subsequent maintenance of the structure, by painting, is thus entirely without electrocution hazard to the men on the job, as they need not go near the live wires. One of the most important problems associated with overhead line supports is that of foundations. For poles in reasonably firm ground wooden blocks, like railway sleepers, laid transverse to the load, are bolted to the stub of the pole. With lattice towers, where tl1e over turning moment may be very great, several types of construction have been adopted. In firm ground, for loads which are not too severe, a rigid flat structure is bolted to the base of each main angle, usually between 6 and 8 feet underground, and a thrust plate may be fixed nearer the ground surface. A more expensive but more satisfactory job, which also preserves the members underground from corrosion, is to encase the foundations in concrete. Firm ground is reduced in effectiveness if so it is disturbed, so that anchors have been developed which can be passed down a small diameter hole bored in the earth with an auger, at the base of which a cavity has been blasted. The anchor is such that it may be expanded within this cavity which is then filled with concrete. A firm anchorage is thereby effected with a minimum of disturbance to the earth. In swampy ground a concrete raft must be constructed for the tower. In this country the minimum allowable distance between a live conductor and the ground is 15-19 feet for low voltages up to 23 feet for higher voltages. In order to obtain the height of a support the sag of the conductors and the total vertical distance between them must be added to this figure, and the length of a suspension insulator must be added or the height of a pin insulator subtracted. The sag depends on mechanical considerations and the distance between conductors on the clearance required for electrical reasons (subject to empirical minima for mechanical clearance). It is usual to include a tensioning structure at intervals even in a straight run of line in order to localise out of balance effects due to the possible breakage of a conductor or a tower failure. Any damage, in such an event, is confined to the section of line between tensioning points. The choice of insulators for any particular transmission scheme depends not only on the line voltage and the normal mechanical loading, but also on extraneous factors due to local atmospheric conditions. The electrical design is affected by the possibility of semi-conducting deposits of dust, fog, salt spray, and moisture forming over the surface of the insulator and the possible incidence of voltage surges due to lightning. The insulator must be capable of with standing stresses due to rapid temperature changes, such as would result from a cold rainstorm falling on a sun-baked line ; and mechanical vibration of the conductors may be set up by winds. A high tension insulator must be absolutely non-porous and capable of resisting a power arc. The most common insulating materials are therefore porcelain, steatite, or special glass which will withstand temperature stresses. The surface of porcelain and steatite insulators is glazed in order to present to polluted atmospheres a surface which is easily washed by rain. The leakage surface across the insulator between conductor and earth must be long enough to be effective even when its resistance is greatly reduced by deposits. Careful design, combined possibly with the introduction of layers of a resilient medium, is necessary to keep internal stresses due to the difference in thermal expansion between the insulating body, the metal fittings, and the fixing cement (if any) within reasonable limits. Considerable trouble was experienced with early cemented insulator fittings, a common cause being the swelling which took place when a mixture of glycerine and litharge was used. The best cement, with which no trouble should be experienced if proper hydration has taken place, is neat Portland cement, or a mixture of Portland cement and sand. The mechanical loading of a line insulator depends on its position. A considerable end tension is applied to line conductors in order to limit the sag between supports. This end tension is experienced only at terminal points and, as a resultant load, at angle points. Inter mediate supports in a straight level run of line encounter only the vertical weight of the conductors and the transverse wind loading on the conductor span. Both these values are increased if ice forms on the line. Pin type insulators may be used at intermediate positions for voltages up to about 6o,ooo, but are rarely used above JJ,OOO volts in this country. Suspension type insulators are common from xo,ooo volts and less up to the highest voltages. The choice of the size and design of an insulator depends to an enormous extent on the climatic conditions under which it will be used. An insulator which may operate satisfactorily at JJ,OOO volts in an equable climate may give · serious trouble on n,ooo volts where it is subjected to atmospheric pollution of a high order. Adverse atmospheric conditions may be extremely localised as in the case of an area very near a cement works. The earliest transmission lines were equipped with pin type insulators, of narrow elongated design, and shackle type tension insulators. Evolution in the pin type insulator has tended towards a widening of the sheds which are also arranged as nearly as convenient at right angles to the lines of force between conductor and earth. Recent progress has been in the provision of long leakage surfaces specially disposed to resist deposits and to benefit by the cleansing action of rain. Cushioning materials, such as lead, are introduced between the steel of the pin and' the porcelain. Hewlett, in 1906, introduced the suspension type of insulator in America. A string of insulators can be built up from any desired number of interchangeable units. A hook or other fitting is connected to the cross-arm unit and a clamp to hold the conductor to the line unit. In order to keep a flashover clear of the units and also to reduce the stress on the line unit, arcing horns or grading rings are commonly fitted to an insulator chain, mainly when the supply voltage is high. If the mechanical loading required is greater than the rating of a single unit, two or more chains of units are arranged in parallel. There are three main types of suspension insulator : i. the cap and pin type in which a metal pin is fixed into a cavity with a ribbed flange, a metal cap being fixed around the outside of the cavity. The ribbed flange provides the insulating surface. ii. the Hewlett type, in which two interlinked metallic U-pieces are separated by an insulating body so designed that there .is a considerable surface along that path between one U-p1ece and the other which is exposed to the air. iii. the " motor " type, used to a considerable extent on the con tinent, in which a shedded insulating rod has metal caps affixed to each end. The first type is the most common, an example of its use being on the 132 kV. "Grid" lines in this country where a suspension string of nine cap and pin type units is generally used, fitted with arcing horns at the earthed end and a grading ring at the line end. The discs are 10 inches diameter and spaced 5 inches apart. Their guaranteed minimum electro-mechanical strength is Io,ooo lbs., and the maximum end loading in the line is 8,ooo lbs. In order to provide an adequate factor of safety two chains are used in parallel where such insulators are used at tension positions. The conductor clamps at suspension positions are designed so that the conductor will slip before too high a load is reached in the event of a breakage throwing the load off a tension point. Insulators of this type can be made with electro mechanical ratings of zo,ooo lbs. and more. Special forms of suspension insulator having long leakage surfaces disposed to resist atmospheric deposits have also been designed. It is necessary, however, periodically to clean the insulators on an overhead line unless conditions are particularly favourable. The development of the grid-controlled mercury rectifier may result in the extensive use of high-tension direct current transmission, in which case some modifica tion in the present standards of insulator design may be expected. A direct current line requires only about two-thirds of the insulation of a corresponding alternating current scheme, and the design of the insulation is not influenced by the problem of capacity. The cost ratio between overhead and underground transmission would be affected considerably by the adoption of direct currents with the possible result of a much wider employment of underground cables in which dielectric and sheath losses would be eliminated, apart from a small resistance leak. The underground system gives greater freedom from atmospheric disturbances, and the reduction in the amount of heat dissipation required might just tum the economic scale in many cases. Serious trouble has been experienced in many localities due to birds short circuiting an overhead line to earth, usually when spreading their wings before taking off in flight. Devices have been adopted to counteract this trouble, some taking the form of insulating slabs for the birds to stand on, others consisting of insulated perches arranged conveniently for the birds, and others depending on making the danger points extremely uncomfortable either by shaping the cross-arms or by the use of jagged metal excrescences. The conductors themselves are sometimes covered with insulating material for some distance on either side of the insulators. Another form of trouble sometimes experienced with overhead lines is insulator or conductor failure, due to excessive vibration. Damping weights are therefore fitted near, or to, the insulator strings in certain schemes and the mouths of suspension and tension clamps are flared in order that the conductor shall not be in contact with any sharp edges. In some cases, also, it is found desirable to attach projecting wire " straw guards " to the line conductors about a yard away from the insulators when these are of the pin type. . These guards retain straws which might be blown across the insulators by the wind. 53 Earthed metal guards are often placed a short distance belQw ijnes so that, in the event of conductor breakage, the line is earthed. and switched out of commission by relay action before it can fall on and damage, by reason of its potential, any structures or beings below. · When a telephone line runs parallel and close to a high voltage line there may be very undesirable induction effects in the telephone circuit. In a long run of line in which the conductors always remain in the same relative position the electrical constants of each phase may differ by a considerable amount. Both these undesira.Rle effects ~y be reduced to negligible proportions by suitably transposing the conductors at regular intervals along the line. Thus, ·if the conductors leave the transformer station in the order I, 2, 3, they are transposed at, say a. the,,· tenth tower, to the order 2, 3, I ; at the twentieth tower, 3, .r._ 2 ; and .. so on. Special arrangements of insulators and cross-arms are required at transposition points. Communications may be transmitted over high voltage lines by means of a " carrier wave " system which may be particularly economical in the case of a long line where the cost of erecting telephone wires would be high. High frequency modulated carrier currents are imposed on the power lines through coupling condensers and are received on an appropriate tuned filter circuit operating through a coupling condenser at the far end of the line. The coupling condensers are fully insulated for the line voltage and are enclosed in shedded porcelain shields so that they are suitable for outdoor use. They may be constructed on the unit principle like suspension insulators, and for " wired wireless " communication purposes their efficiency is greatly superior to that of the earlier antenna couplers consisting of an aerial slung below the power line. They may be used, in addition, for providing voltmeter or synchronising supplies, and may even be economical for tapping a small power supply from a main line. To face page 54, FIG. 26.-Ferranti Paper Insulated Cable, 188g. FIG. 27.-Edison Electrolytic Meter, 1881. FIG. 28.-Ayrton and Perry Polarised Iron Ammeter, 188o FIG . 29.-Ferranti-Wright Meter. 1890. (showing a spare magnet). INDUSTRIAL ELECTRIC MEASUREMENTS Measurement is essential to progress and for an appreciation of the economic factors in electrical development. This was recognised as early as r86r, when the British Association Committee on Electrical Standards defined a logical system of units depending on primary standards of measurement which were settled according to an arbitrary physical specification. Such standards have now been agreed upon internationally and may be constructed with an accuracy of a few parts in a million. For ordinary measurements, however, such accuracy is not required, and instruments have been devised to give direct readings of sufficient accuracy. They are calibrated in relation to sub standards, or instruments of very high accuracy and convenient range, . which are themselves calibrated in regard to the primary standard. The instruments for industrial use must be cheap, reasonably accurate, and very robust. The first industrial instruments were often zero reading, with scales marked in degrees, and requiring the use of curves or constants to obtain a result. Ayrton and Perry, in r88o, introduced a direct reading commercial polarised iron instrument 1 (Fig. 28) which elimi nated such delay. In those days current and voltage were practically the only readings expected apart from the ampere-hour readings on which power consumption was charged by the few pioneer supply companies which were just starting operations. The demand for further knowledge and higher 11ccuracy rose steadily as distribution schemes became more ambitious. Ampere-hour meters were sup- . planted to a large extent by those reading watt-hours, and intermittent recorders were replaced by movements capable of continuous rotation. Crompton realised the importance of "load factor," and Wright, recognising this principle, introduced the " maximum demand " system of charging for power, and invented an instrument, in r8g3, which would register such a demand."" The growth of alternating current systems created a demand for numbers of new instruments to register power factor, phase. rotation, and the moment for synchronising, etc. The development of the instrument makers' art was reflected in the continual discovery of new materials and methods whereby moving parts became smaller and lighter and gave a better torque, and errors were reduced by the introduction of compensating devices. The result to-day is a compact and accurate instrument at a reasonable price and of sturdy construction ~or. al~ost every conceivable requirement while, if necessary, the md1cat10ns may be recorded, not only at the site of the instrument itself, but at any desirable position up to hundreds of miles away. ~uch remote indications are particularly useful in the case of large mter-connected transmission systems and where automatic substation control is required, or to acquaint the boiler house staff with the load being carried by a station. · 55 In spite of the multiplicity of uses for which an instrument may be ' designed the action of every electrical instrument, except those of the electrostatic, electrolytic, and thermal types, depends on the creation of a magnetic field by passing a current through a solenoid. According to the way in which the solenoid is connected to the circuit its field will bear some relationship to the current or voltage from which it is desired to obtain a measurement. This field may be made to interact with other fields, similarly produced, or with a constant field due to a permanent magnet, in order to give a resultant field which will affect the position of a movable system to which a needle or registering mechanism may be attached. Ammeters and voltmeters of any one type are, in principle, the same instrument, except that the actuating circuit is designed for inclusion in series with the main circuit in the case of ammeters and in parallel in the case of voltmeters. The resistance of a voltmeter coil is therefore high compared with that of an ammeter. Wattmeters, measuring the product of current X voltage ( X power factor in the case of A.C. circuits) are equipped with both a voltage coil and a current coil. Supply meters, measuring kilowatt hours, are in effect wattmeters in which an integrating wheel train replaced the needle and the move ment is allowed to rotate instead of being restrained by a spring. In ampere-hour meters the voltage of the supply is assumed to be constant for the purpose of charging for the units consumed, and, if they are to give an approximation to the total kilowatt-hours, may be used only on D.C. circuits. The action of an instrument may be dependent on factors other than the actual principle of the movement. In certain classes of instrument use is made of two circuits in parallel, one of which is made highly inductive and the other highly non-inductive. Two actuating coils rigidly fixed at right angles are connected in these circuits, and in the case of change in the frequency of a supply the ratio of the currents in the two circuits will alter, displacing the resultant flux vector between the coils and consequently deflecting a needle connected to the moving system, due to interaction with the flux from a fixed coil. Frequency meters may therefore be constructed on this principle. If the phase of the voltage applied to such a parallel coil circuit differs from that in a fixed current coil one of the moving coils will experience a higher torque than the other and a deflection may be obtained depending on the power factor of the main supply. Synchroscopes as well as power factor meters may also be constructed on this principle. Frequency meters may also be constructed depending on the resonant vibration of iron reeds tuned to various frequencies and actuated electro magnetically. The reed corresponding to the supply frequency will vibrate with the greatest amplitude. The sources of error in instruments are due to various causes such as: 1. Pivot friction, which is reduced to a minimum by using jewelled bearings and the lightest possible moving system. The design of s6 · current carrying .contacts to the moving system must be considered in relation to the minimisation of friction. One of the most striking: points arising from a comparison between early and modem instru- . ments is the way in which inertia of the moving system has been reduced progressively as new materials and methods became known. 2. Hysteresis may occur in direct current soft iron instruments and results in a difference between readings taken with increasing and decreasing values of currents. In the earliest instruments this difference was often as much as :z or 3 per cent. It may be minimised by working at low magnetic intensities and using particularly soft iron. 3· Temperature variation may alter the resistance of the whole, oi part, of the instrument circuit including external apparatus such as shunts and instrument transformers. Control springs may also be affected. 4· The influence of external magnetic fields may be experienced by all magnetic instruments unless screened. If the movement works in a strong permanent field the effect of external fields is very much less marked. S· Levelling. In instruments having gravity control or suspended movements, and in thermal instruments, accurate levelling is essential in order to reproduce the conditions under which the instrument is calibrated. 6. Eddy currents induced in metallic masses and around the movements of alternating current instruments. 7· Frequency and wave form errors in • alternating current instruments. 8. Weakening of permanent magnets and control springs. 9· Self-induction. 10. In supply meters other than those working on the electrolytic principle an initial starting torque is required to overcome friction so that registration may not occur on very low loads. Instruments and meters, when the cumulation of the errors to which they are susceptible renders their accuracy below that required for the purpose for which they are intended, may be compensated by the inclusion of devices having an effect contrary to the error. Almost all errors can be compensated, but it may be more economical, given a certain set of operational conditions, to employ an equivalent instrument actuated on a different principle rather than compensate an instrument of any particular type. In the following paragraphs a classification of instruments according to principle has been made. MOVING MAGNET INSTRUMENTS Oersted, in 1820, discovered that a conductor carrying a current ~xerted a force o~ a ?eighbouring magnet. This principle is employed m electromagnetic mstruments of the moving magnet type. It is common for a pair of magnets to be mounted astatically, one inside the 57 operating coil and the other outside, so that the effect of the earth's magnetic field is cancelled. This arrangement was first introduced by Nobill, in 1825, as a refinement on Schweigger's " Multiplier," or galvanoscope, of 1820. In the" Multiplier" the importance of having a coil of many turns of wire was first appreciated. Although the moving magnet system was prominent in the earliest days of electrical engineering it has now given way to other classes of instrument in all but a few commercial applications. MOVING IRON INSTRUMENTS Arago and Davy, in 1820, discovered that if a piece of unmagnetised iron was substituted for the magnet of Oersted's experiment a force would be exerted between a conductor carrying a current and the piece of iron. Schweigger found in 1821 that the influence of the current was magnified by passing it through a coil of several turns, and in 1824 the electromagnet was discovered by Sturgeon,"" being vastly improved by Henry in 1828.• These experimenters laid the foundations on which the moving iron type of instrument has been built, a direct pull type of ammeter having been introduced by Kohlrausch as early as 1876. Electromagnetic instruments of this type may be classified as follows: Attraction Instruments. (a) Direct Pull 'type. A saft iron plunger is attracted axially into a solenoid carrying the current to be measured (e.g. Kohlrausch, 1876; Blyth, 1882 ; Ayrton and Perry,"" 1883, etc.). (b) Deflected Needle Type; in which a soft iron needle is pivoted so that it tends to set itself along the field of a magnetising solenoid against the action of a spring or control magnet (Ayrton and Perry, x88o,"" Miller,• x884, Siemens and Halske, etc.). (c) Eccentric Needle Type. The needle is parallel to the axis of the solenoid but mounted eccentrically so that deflection brings the iron into the stronger field close to the winding (Schuckert). (d) Attraction Iron Type. The iron is attracted towards the gap between the ends of one or more irons placed so as to complete the field of the coil (e.g. Evershed and Goolden, x887,• Stanley). Repulsion Instruments. The moving iron is repelled from a similarly charged iron lying parallel to it within the coil and parallel to the axis of the coil (e.g. Nalder, x885, etc.). Combined Repulsion and Attraction Instruments. The moving iron is repelled from a parallel iron and attracted towards the gap in another iron as in (d) above (Harrison, Holden, etc.). ss Moving iron instruments have been manufactured extensively for alternating and direct current indications, their cheapness and ability to withstand rough usage being important factors. In the simplest forms they have a poor scale, very crowded at the zero end, but in recent designs this disadvantage has been overcome. Correction for temperature errors, in the form of a non-inductive resistance of low temperature coefficient, is commonly employed with voltmeters in particular, and care must be taken that the instruments are subjected as little as possible to stray magnetic fields. Hysteresis and eddy current errors can be overcome largely by careful design, while a frequency compensation circuit is introduced as an auxiliary by many makers. MOVING CONDUCTOR INSTRUMENTS Oersted, in 1820, discovered that the converse of his first classic experiment held true, namely, that a current carrying conductor would move in a magnetic field. In 1823 Davy showed that mercury, carrying a radial current and subjected at the same time to an axial magnetic field, would rotate. Barlow in the same year substituted a copper disc for the mercury, and a year later Arago discovered the braking action of a magnetic field on a copper disc. From these experiments a number of developments in the form of galvanometers, etc., were introduced by physicists, and in 1883 Ferranti introduced a D.C. ampere-hour meter* depending for its action on the rotation of mercury. In 1887 Hookham introduced a commutator motor meter * employing an eddy current brake, and some years later Elihu Thomson, by eliminating iron from the core, constructed a similar instrument suitable either for A.C. or D.C. circuits.* Com mutator motor meters of less practical importance were devised by several earlier investigators, including Ayrton and Perry, 1882, and Hopkinson. In 1888 Dr. Weston, adapting the Kelvin Syphon Recorder and the D'Arsonval galvanometer, introduced a practical permanent magnet moving coil voltmeter for industrial or laboratory use. The permanent magnet moving coil instrument is the most accurate and useful type of instrument for direct current measurements, giving a good torque, an even scale of great length if required, flexibility of range through the use of appropriate shunts and resistances, excellent damping, and freedom from serious hysteresis, temperature, and stray field err?rS. The permanence of the magnet may be ensured by careful design and ageing, and the chief disadvantage of the type is that the construction is more costly and susceptible to injury than in the moving soft iron type. Other. instances of moving conductor instruments include the Duddell oscillograph, t8g8, and the Braun oscillograph of 1897· The latter, in which the conductors are charged electrons, has developed into the cathode ray oscillograph essential for research into high frequency phenomena, while the former, in improved form, is used extensively for recording wave shapes on lower frequencies. 59 ELECTRODYNAMIC INSTRUMENTS Ampere, in 1820, demonstrated that a current carrying conductor exerts a force on a neighbouring current carrying conductor. From this discovery the electrodynamic instrument in which the conductors are in the fonn of adjacent or concentric coils has been evolved, a large electrodynamometer having been constructed in 1861 for standardisa~ tion work by the B.A. Committee on Electrical Standards (after Weber, 1846). The current balance of Joule, introduced in 1866, was also notable amongst instruments of this type constructed before the introduction, by Siemens in 1877, of a zero-reading electro dynamometer for industrial use • which became widely adopted. A direct~reading dynamometer voltmeter was introduced by Weston in 18go. In 1882 Profs. Ayrton and Perry made an instrument from which power consumption could be calculated according to the loss of time of a clock, • the pendulum of which carried one coil oscillating inside the other coil, which was fixed. This construction was later modified into a differential registration of the rate of two pendulums in the Aron meter (1886),• which is still used, mainly for bulk metering, for the direct recording of power consumption. Dynamometer instruments are used mainly as standards, being free from errors due to hysteresis or change of magnetisation and giving exact readings with alternating currents provided that the coils are kept away from metallic masses which might introduce eddy current errors. The torque is proportional to the square of the current so that, unless special devices are adopted, the effective range of such instruments is low. Dynamometer ammeters for standardising are more difficult to construct than voltmeters and wattmeters as, if the moving element is shunted, there is a contingent liability to error, while the introduction of a heavy current to the moving coil, if unshunted, presents equal difficulty. In the case of voltmeters and wattmeters the current in the moving coil need be only a few milliamperes. In 1904 Dr. Sumpner introduced an iron-cored dynamometer,• the action of which, when used with a quadrature transfonner, resembles closely that of an induction type instrument. ROTATING FIELD (INDUCTION TYPE) INSTRUMENTS Arago, in I 824, demonstrated that a conductor would tend to follow the motion of a rotating magnet. In 1879 Walter Baily, before the Physical Society, showed an experiment in which the polarity of two alternate sets of electromagnets was successively reversed by turning a hand commutator device connected to a direct current supply.• The result was to produce in effect a rotating magnetic field from stationary magnets, and a copper disc, pivoted above the magnet faces, rotated also as the commutator was turned. Ferraris produced 6o the same effect by purely electrical means in 1885, using an alternating current supply to two coils with their axes at right angles, one coil being supplied with current 90° out of phase with the other. Induction type ammeters and voltmeters are usually constructed on this principle. An eccentrically pivoted disc or cam is at right angles to the axis of an alternating magnet which induces a current in the movable element. The reaction between the induced current and the inducing field tends to demagnetise the magnet, and the element therefore tends to move out of the field of the magnet to allow the lines of force to pass. A deflec tion is thus obtained. Ferraris suggested the application of his discovery to measuring instruments and an induction motor meter was made by Borel and Paccaud in 1887. In 1887 Prof. Elihu Thomson found that if two or more alternating magnets are used the element tends to rotate due to a" repulsion effect." The eddy currents induced in the movable element by one magnet pass across the field of the second magnet and vice versa, thereby tending to make the element rotate according to the same directive laws as apply to any current carrying conductor in a magnetic field. Wattmeters, supply meters, and phase indicators belong mainly to this class of instrument. In x888 Shallenberger introduced one of the earliest successful supply meters of the induction type,* and in 1889 A. Wright* dis covered that if a conducting plate is interposed between the moving element and part of a single magnet core, or part of the core is sur rounded by a short circuited ring, the result is the equivalent of a two pole instrument, described above, as the fields from each portion of the core differ in phase. " Shaded Pole " instruments have developed from this invention, one of the earliest commercial applications of the principle being the Ferranti-Wright meter of x8go * (Fig. 29). All induction type instruments are suitable for use on alternating current circuits only. The moving element, which usually takes the simple and robust form of a copper or aluminium disc, is free from current carrying contacts, and therefore from any great frictional losses. The position of the magnets in relation to the disc does not demand great accuracy in construction, and eddy current damping, by means of permanent magnets, can be applied conveniently to the same disc. A high torque/power ratio and a long scale in the case of indicating instruments are readily obtainable. Induction instruments are subject to frequency errors and a number of small hysteresis, eddy current, and other errors which normally can be compensated during manufacture. In the case of integrating instruments the temperature errors, which may be considerable owing to variation in resistance of the moving element, are cancelled automatically by variation in the braking torque. The accuracy obtainable from induction instruments is usually of a commercial order only, but, for alternating current service meters, they are used almost to the exclusion of any other type, and the indicating instruments are used widely for switchboard work. 61 RESISTANCE MEASURE~IENT The direct measurement of resistance, particularly insulation resistance, is of great importance to the electrical engineer, and many instruments have been de\ised for this purpose, most of which depend for their action on the ohmmeter principle invented by Ayrton and Perry in 1881.• Two coils, within which a small permanent magnet is pivoted, are arranged with their axes at right angles. In the circuit of one coil is inserted the resistance to be measured, while the other coil carries a current proportional to the applied voltage. Ohm, in 1826, defined the law that voltage equals current multiplied by resistance, and the position taken up by the needle of the ohmmeter is along the resultant field which depends on the ratio between the applied voltage and the current through the unknown resistance. The deflection of the needle can thus be calibrated against resistance. The "Megger," designed by Evershed, and including a magneto generator as a voltage source in the instrument case, is a good example of the convenient portable form in which such instruments are now supplied. It originated in his combined generator and ohmmeter of 1889, was improved by the inclusion of an astatic moving system in 1890, and in 1894 the magnet was replaced by a soft iron needle.• In I 904 a moving system similar to that used in a moving coil ammeter or voltmeter was adopted, and the ohmmeter and generator were combined in one case. The following classes of instrument depend for their operation on principles other than electro.magnetic : ELECTROSTATIC INSTRUMENTS These instruments depend for their action on the forces exerted between two electrified bodies, and they originated with the simple electroscopes and electrometers devised by the early experimenters in frictional electricity. These instruments were not at first quantitative, and, after crude forms of electrometer had been introduced by Bohnen berger, Henley, and others, Lord Kelvin in 1856 produced his divided ring electrometer.• This instrument developed into the quadrant electrometer which, in 1887, was put into commercial form as an indicating electrostatic voltmeter for use on pressures between 400 and 1o,ooo volts. In 1890 the sensitiveness was increased by making it multicellular so that a range from 6o-I so volts could be obtained.· The Snow Harris attracted disc electrometer of 1834 was improved by Kelvin into an "Absolute Electrometer" in 1852. It enabled an applied voltage to be deduced from the dimensions of its parts and the force of attraction without reference to any other standard. It was pointed out by Potier, in I88I, that if the needle of an electro meter were connected to one side of a circuit and the plates to a resistance across the other side, the instrument would measure watts 62 accurately even with inductance or capacity in the circuit. Mter a number of experiments, which began in 1884, such an instrument was finally constructed satisfactorily by Addenbrooke in 1900, * and improved by Patterson and Rayner in 1913. They must be permanently installed and constitute very delicate reflecting instruments for use in the standardisation of alternating current instruments and have been used extensively with important results in measuring dielectric losses in cables, etc. Mention should be made of electrostatic leakage indicators and the Cox electrostatic ohmmeter. These instruments were devised for ordinary commercial protection and testing, but, in general, the electrostatic principle is particularly useful in applications where extreme accuracy is required and under conditions of delicate control. The low working forces and freedom from hysteresis, wave-form, frequency, and temperature errors are advantageous, while disturbance of circuit conditions due to the instruments themselves is minimised. Electrostatic instruments are equally applicable to alternating or direct current measurements. The ionic wind voltmeter {Thornton, 1930) is a special form of electrostatic instrument for use on high voltage alternating current systems. The cooling of a wire under the influence of molecules to which motion has been imparted by ions repelled from a highly charged point is made to indicate the extent of the charge. ELECTROLYTIC INSTRUMENTS Faraday, in I833, proved that the amount of a liquid compound decomposed by a current of electricity is directly proportional to the absolute quantity of electricity passed. The first service meters used on a public supply system were based on this principle, Edison 11 having in z88r adapted the zinc voltameter for use on his Pearl St. undertaking in New York (Fig. 27). The gain in weight of a zinc cathode was used as a measure of the·current consumed. The Wright mercury voltameter,• introduced by Wright in 1900, was a great improvement on that made by McKenna in 1892, and is one of the few electrolytic meters which survive to any extent to-day, as in this type no renewals are necessary and chemical permanence can be ensured. Electrolytic meters are cheap, simple, and reasonably accurate, but they usually demand a fairly large voltage drop across the meter, the use of fragile glass parts, and destruction of the old record after resetting. There is no friction, no magnetic circuit, and no stray field effect, but temperature compensation is essential and mechanical shocks must be avoided. Provided that precautions are taken, there is little reason to expect chemical change in the electrolyte and electrolytic meters begin to record on the smallest currents. 6J THERMAL INSTRtThiENTS Joule, in 1839, demonstrated that the rate at which electrical energy is dissipated as heat from a conductor depends on the current flo\\ing and the resistance offered to its passage. In 1883 Cardew • constructed a voltmeter in which the pointer was deflected by the expansion of a \\ire through which a current flowed proportional to the voltage to be measured. This instrument was followed by others, similar in principle, but in which the sag of a heated wire was measured (e.g. Hartmann and Braun, 1891, Holden,• 1893). Klemencic, in 18gr, applied a thermo-junction to a heated wire in order to measure the current in the latter. The thermo~junction was a discovery by Seebeck, in 1821, to the effect that if two junctions of dissimilar metals in a circuit are at different temperatures, a potential depending on the difference in temperature will arise between the two junctions. Such potentials can be measured by an ordinary milli~ voltmeter. The advantage of such instruments is that a thermo~ junction can be calibrated against the current supplied to the heater at lowfrequency,and this calibration holds good also at radio frequencies while the millivoltmeter used is a direct current instrument. Hot wire instruments can be constructed in such a manner that they are almost entirely non~inductive, a point of great importance in measuring radio-frequency currents, but they are usually sluggish in action and highly susceptible to overload damage. They must be . effectively screened against air currents and must always be used in the exact position in which they were calibrated initially. The hot wire and thermo~junction are often· enclosed in an evacuated bulb in order to secure greater constancy of conditions. This construction was introduced by Lebedew in 1902. For currents greater than about 20 amperes hot wire instruments become too sluggish in operation and very wasteful in power, so that current transformers or shunts are normally employed. For radio frequencies these must be specially designed. Thin metal strips of high resistivity are commonly used instead of wires in shunts for high frequency measurements in order to reduce " skin effect," and the disposition and matching of the strips in the case of a number of parallel conductors is important. Errors due to hysteresis, wave form, stray fields, and frequency are practically absent from hot wire instruments, but they often have an uncertain zero and the scale shape is usually poor. They are not precision instruments, but their accuracy is within reasonable commercial limits if proper preca~tions are taken. STORAGE The storage of electricity in bulk is, in general, uneconomic, with the result that the generating capacity of an electric power station must be about three times as great as the average demand upon it. It is often necessary, however, for a supply of electrical energy independent of supply mains to be available, or it may be economical to use a certain amount of energy stored during low load periods to supplement the generating capacity of a station during peak demand periods. The secondary cell provides such supplies. Theoretically there is little difference between a secondary cell or accumulator and a primary cell. Each is an apparatus for the conver sion of chemical into electrical energy, and the practical difference chiefly resides in the degree of reversibility of this process within the cell. Mter discharge the secondary cell can be regenerated by passing a current through it, and by this means so nearly restored to its original chemical condition as to enable the process of charge and discharge to be repeated very many times without serious loss in efficiency. In addition to this advantage, the chemical behaviour of the materials chosen for use in secondary cells is such as to enable them to be made with a capacity suitable for industrial purposes and discharge rate which is large when compared to that of a primary cell. The first primary cell was made by Volta in 1799. In z8o1 Gautherot found during some experiments in electrolysis that the arrangement of platinum and silver wires dipping in salt water was capable, after a current had been passed through it, of itself giving a current for a short time. Two years later J. W. Ritter, a German physicist, constructed a cell consisting of a number of discs of copper, separated by pads moistened with a salt solution. The experience gained by the work on primary cells of Daniell, Grove, and Bunsen made it clear that it was the effect of the oxygen gas liberated at the positive plate and the hydrogen at the negative that enabled the simple arrangements of Gautherot and Ritter to give a " secondary " current. Grove, in his "gas " battery in 1842, used plates of platinum strip coated with a layer of spongy platinum surrounded by hydrogen and oxygen, but the cell was of small capacity. As at this time primary cells were the only available means of supplying current there was no immediate demand for secondary cells and little progress was made until the French physicist, Gaston Plante, commenced his researches. Plante used a vessel containing an acid solution, and after experimenting with various metals he discovered that lead electrodes in a solution of dilute sulphuric acid were most satisfactory. The results of his researches were embodied in his book published in 1879. He found that the effect of the charging current was to liberate oxygen at the positive plate which combined with the 5--(261) 6s lead forming a coating of lead peroxide, while hydrogen was released at the negative plate. During discharge the lead peroxide was changed into lead sulphate. Plante's first cell was a glass cylinder containing dilute sulphuric acid in which were two lead plates separated from each other by rubber bands and rolled into cylindrical form. Although the efficiency of the cell was low at first Plante discovered that it could be enormously increased by the process known as cc forming." This consisted in chargipg the cell in one direction for a short period and then discharging it. There followed a charge in the opposite direction and for a longer period. The process of charging in alternate directions for lengthening periods, with a discharge between each, was continued for some time, until the surfaces of the plates assumed a spongy nature with considerable increase in effective area. This, however, was found to be an expensive and lengthy process, and the attention of inventors was directed towards a method of cheapening it. In x881 Carnile Faure patented the method of" forming" plates by coating.them direct with a thick layer of a red oxide of lead without going through the tedious operation of forming this layer upon the plates themselves. In one of his early types of cell the oxide was mixed into a thick paste with dilute sulphuric acid and spread upon the plates, which were then separated by cloth or felt, rolled into a spiral, one within the other, and arranged in a trough containing acid. By this advance the cell was not only ready to receive a charge without the process of forming, but possessed a far greater accumulative capacity than the unpasted Plante cell. The tendency of the layer of oxide to become separated from the plate was a serious disadvantage: The felt was found to rot away under the action of the acid, and separators of other materials were found to be equally unsatisfactory. In the same year, however, these difficulties were largely overcome by Sir Joseph Swan, who invented the lead grid or reticulated plate with a number of interstices into which the paste was packed. Since the date of this introduction a large number of different forms of grid have been produced by inventors, the object of which is to keep the active material in close contact with the support. MODERN LEAD CELLS In some cells both the negative and positive plates are pasted, but for quick discharge and for such purposes ·as train lighting and traction, the Plante form of unpasted positive, formed by an improved process, is often used. Semi~ Plante, or partially pasted positives, are sometimes employed. For stationary work containers are of glass, or wood lined with lead. Ebonite, glass, or celluloid are used for portable cells. Mter charging, the voltage of a lead secondary cell at normal discharge is 2 volts. This value is maintained until the cell is half discharged, the voltage then gradually falling to about I ·8 volts when fully discharged. The efficiency of a secondary cell may be given 66 either in terms of quantity or energy. The former or ampere-hour efficiency varies from 9C>-95 per cent., the watt-hour efficiency from 72-78 per cent. These are dependent upon the maintenance of the cell in proper condition. Attention must be given to ensure that the rates of charge or discharge do not exceed the allowable values which are controlled by the size and number of plates. A usual rate of .~s charge is from 4-5 amperes per square foot of total surface of pos1t1ve plates, but this rate may be considerably exceeded if necessary. CELLS OTHER THAN LEAD_ The disadvantages of the lead cell are chiefly its low voltage and its heavy weight. Many attempts have been made to use lighter materials, if possible increasing the e.m.f. and giving a greater capacity per unit weight. Thomson, Houston, Reynier, Edison, Lalande, Epstein, and others made experiments with this end in view. The possible com binations of metals fulfilling the required conditons are, however, very few, and of these only the iron/nickel oxide alkaline cell, the zinc/lead peroxide acid cell, and the zinc/copper oxide alkaline cell have been put into practical form The last two are unsatisfactory, partly on account of the behaviour of the zinc and the extreme readiness by which it undergoes oxidation by local action. In the zinc/copper oxide cell the electrolyte is very sluggish, preventing proper diffusion, and it absorbs carbon dioxide from the atmosphere thereby retarding its activity. Dr. Ernest Valdemar Jungner had been experimenting with this type of cell in order to overcome its defects, and in 1899 he patented a cell in which the electrolyte remained unchanged. The liquid consisted of a solution of potassium hydrate and acted only as a con ductor. Jungner mentioned the use of powdered silver peroxide pasted on a nickel wire net as the positive, and a copper wire net into which finely divided copper was pressed as the negative plate. This combination gave from o·95 volt to I volt. A number of pairs of these plates was used, asbestos paper soaked in the alkali hydrate solution being placed between them. The container was of ebonite or other suitable material. The combination of iron , and magnesium oxides was also mentioned. The Nickel-Cadmium-Alkali cell has been developed since the original patent of Jungner, and to-day a cell of very low internal resistance, enabling it to give the heavy currents necessary for switch tripping, motor-car starting, etc., can be made by inserting strips of pure nickel corrugated strip in the pockets of active material. This refinement was introduced into the" Nife "cell in 1929. A number of experiments on similar lines were carried out by T. A. Edison, who took out several patents in 1900 and 1901. In his 1900 patents he specified plates of nickel or iron suitably plated. Finely divided copper was the negative element and finely powdered cadmium 67 the positive, both being supported in their respective plates by suitable pockets. Later he used nickel hydroxide for the positive and iron oxide for the negative. Edison continued his investigations, and about 1908 evolved a practical form of this cell. The iron/nickel oxide alkaline cell is lighter for a given output than the lead cell, but has a lesser voltage, varying during discharge from 1·4 volts to 1·1 volts. The ampere-hour efficiency is about 8o per cent. and the watt-hour efficiency 55--()o per cent. The lightness of the cell makes it suitable for electrically propelled vehicles, it is mechanically strong, and large currents can be taken from it without damage. The Drumm cell, which has more recently been introduced in Ireland, is being used in electric traction schemes, as it may be charged up and discharged with extreme rapidity without injury to the plates. An electric train can thus be run on a non-electrified system if arrange· ments are made for intensive battery charging during stops at stations. The battery consists of a positive plate system ofthe Ni(OH)2-Ni(OH)3 mixture first developed by Edison, but the negatives are nickel gauze grids immersed in a solution of zinc oxide in potash. The voltage is about 1 ·86 volts per cell, or about 40 per cent. more than that of the nickel-iron accumulator. The charging rate may be about four times that of other alkaline cells, and the normal discharge rate about twice as great although, for limited periods, rates of discharge up to five times that recommended for nickel-iron cells may be used. The watt-hour capacity of a Drumm cell on a single charge is low, but its capacity for high loads enables as many as twenty charges to be applied and dis· charged during a working day, thereby making the daily output per unit weight of battery about three times that of the ordinary alkaline cell, given the necessary charging facilities. The average current efficiency is 92-93 per cent., and the average voltage during charge and discharge 2·03 and 1·65 respectively, giving an energy efficiency of about 75 per cent. 68 APPLICATIONS OF ELECTRICITY The development of apparatus for the production and distribution of electricity has been concurrent with the expansion of the services to which it could be applied. The demand may have arisen, and apparatus have been evolved to meet it, or a new invention or a design of increased capacity may have stimulated a demand for new methods of application. The magneto-electric machine was adapted by Clarke in 1835,'" to the somewhat doubtful service of providing a supply for " medical coils." Later, in 1838, the invention of electroplating by Jacobi created a demand for steadier and stronger currents, and Woolrich, in I 84- In general, the use of electric power for main lines is dependent on the cheapness of the available supply. The electric locomotive is useful where heavy gradients may be encountered, and, in mountainous country, may return as much as 20 per cent. of its total consumption of power by regeneration. on .down grades. The alternating current locomotive is usually designed to have two or more fixed speeds, and a low frequency of supply is desirable (say 15 cycles). The Kando system, employed in Hungary, makes use of single-phase, so-cycle supply fed to a pole-changing traction motor through a phase-changing machine. The extra cost and complication of the equipment must be 7'1. [T o face page 72 . [Metropolitan- Vickers Elec trical Co., L td. F I G. 30.- Stator of 2,410 h .p ., 43 r.p .m., \\"inding - Iotor . [C. A. Parsons & Co., Ltd. FIG. 3 r .-Transport of a large Alternator Stator. To /act pagt 7 J. j FIG. 32.- Sectioned :\lode! of a H eroult Electric Furnace. FIG. 33.-Electric Locomotive. Ciry and South L ondon Railway, 1890. The general public is gradually becoming more and more electri cally minded. Wireless communication, telephony, and domestic appliances are now not only used, but are to a certain extent understood by the majority. When it is realised that, considerably less than a century ago, the possession of an electric bell was unusual, the pheno menal growth of electrical engineering can be better appreciated. A non-technical legislative body would have been hesitant in co-ordinating the electric supply systems of Great Britain by the establishment of the Central Electricity Board had it not been clear that the prosperity of the community is closely linked up with electric power development. Progress in Britain was greatly impeded in the early days of electric power supply by restrictive legislation which was not repealed until nearly the end of the last century. Public supply companies could only obtain rights for a very limited period of years which made the sinking of capital in such ventures an extremely hazardous procedure. This, combined with the suspicion with which electrical supply was regarded by the fire insurance companies, in those days with a certain · amount of justification, arrested development in this country to an enormous extent at a time when great activity was being shown on the Continent and in America. The standards of safety required by British regulations are at present more severe than those of many other countries, with consequent effects on the cost of distribution. There is divergence of opinion on the question of their desirability as many schemes of rural distribution and other border-line loads are held up through economic factors accentuated by such regulations. On the other hand, there is a commercial, as well as moral value to be placed on safety. It is impossible, in a short chapter, to convey adequately the rami fications of the electrical industry, but, in conclusion, a few statistics may be given in order to show the enormous growth during a relatively short span of years. 1. PROGRESS IN GREAT BRITAIN (a) Estimated output in millions of kilowatt-hours Authorised Electric railways Private industrial Total undertakings and tramways plant 4.734 433 3,667 8,834 ~0 11,120 1,250 •• :zoo 16,570 (b) Generating Capacity in thousands of Kilowatts Public supply Private supply Total 1920 z,6:z6 2,282 4,908 1930 6,6oo J,Q40 g,640 75 (c) Value of Electrical Manufacturing Production in £1,000 1907-8 1930 Electrical machinery •• 17 10<)1 19,176 Control and switchgear • • • • 3.538 7.593 Wires and cables • • • • • • 18,4+4 19,192 Wireless apparatus .. 6,632 ?,426 Accumulators and batteries .. 4,298 5.749 Total machinery and apparatus •. 70,653 77,000 Note.-The figures for 1932 show a considerable falling off due to the world-wide economic depression and the fall in prices. The above figures illustrate nonnal progress to a reasonably comparable degree. (d) Number of workers employed directly 1923 . . 57,807 1924 • . 68,064 1927 . . 75,510 1930 • . . . 82,776 1932 . . • • 78,917 2. WORLD PRODUCTION OF ELECTRICITY (millions of kilowatt-hours) 1925 • . 186,595 1926 . . 204,836 1927 • . . . 23J,407 1928 . • . • 269,400 I 929 • • • • 299,400 1930 304,100 1931 .. .. 306,ooo (Figures supplied by the British Electrical and Allied Manufacturers' Association.) NoTE.-Part II of this handbook contains catalogue descriptions of the objects in the electric power section of the Science Museum. INDEX PAGB PAGB Accumulators • 6s Ferraris • • 21, 6o · Acheson • 70 Fleming, Sir I. A• . 23 Adams • 11,20 Forbes • • 101 II Addenbrooke • 27,63 Frequency control 17 " Alliance " machines • 8,9 Furnaces, electric • 70 Alteneck, von Hefner • 9 Fuses. • 42 Alternators • 11-16 Amp~re • s. 6o Ganz & Co. u, IS Arsgo • • s. s8, 59, 6o Gautherot • • • • • 6s .Armature, drum 9 Generators, alternating current II-16 -.ring • 9 -, direct current 7-U -,shuttle • • 9 -. high voltage • IS Arrestors, lightning • 42 -,hydro-electric . .... Atkmson • • • • • 23 -,turbo- • • , 13, 14 Ayrton, and Perry s5,s8,s9,6o,6a Gibbs. Gaulard and , 101 z6, 46 Gorges • 27 Baily, Prof. W. • 21, 6o Gramme • • • , 91 101 II Barlow • • • • 59 Grid-controlled rectifiers • 32 Bemados and Slavionoff • 71 Grondahl • 33 Birkland and Eyde • • 71 Grove • 6s Blathy (see Zipernowski) Hadfield, Sir R. • Borel and Paccaud • 61 • Z7 Brags tad Harmonic absorber • 3 (261) Wt. 354-2832. 2,500, 7/33. W. C. & S., Ltd. Gp. 301. [British BroadcaJtin11 Corporation. FIG . 1 .-Brookman's Park Twin Transmitters.
Faraday, I791- I867. ELECTRIC POWER Part !-History and Development.'
