March 10, 1882. THE ENGINEER. 169

obtained or considered, though a most important element. hesitation to put a lighted lamp into the middle of a powder barrel. STEEL FOR STRUCTURES. Now the comparison of these four sets of figui'es for a We wish to emphasise the assertion that the accidents that have Amongst engineers and constructors there seems still to happened, or which may happen, are due entirely to insufficiently number of different specimens or makes of steel is not very careful engineering. be a good deal of difference of opinion as to what should easily made so that the best relation of the four values is Sufficient has also been done in the practical application of elec­ be the limits of strain which may be visited upon steel in clearly conceived. It is desirable that they should be so tricity to lighting purposes to indicate the directions in which engi ­ structures, and more especially such structures as bridges collated as to express in one quantity or index the value neers and electricians must work in order to arrive at the most of the material for work under tension and in one quantity economical and most complete systems of electric lighting plant. and roofs. A paper on steel for structures was read Much of the progress which has been made toward a large use of before the Institution of Civil Engineers on the 1st inst. the value for work under compressive strain. Having electricity for the purpose has been effected by engineers and prac­ by Mr. Ewing Matheson, the value of which was greatly brought all the values down to a pair of indices in this tical machinists who have turned their attention to the subject. enhanced by the important discussion which it provoked way, the relative values of iron and of steel could be so The electrician alone has moved slowly, and the engineer would then and on the 8th inst. The chief object of the paper simply and clearly shown that the demonstration of the move even more slowly, but the combined labours of the members uncer- desirability for relaxing the official rules would be a com ­ of these two professions have brought about a sufficiently wide was to draw attention to the fact that though the application of electricity for lighting purposes to enable us now to tainties and want of uniformity in steel, which hindered paratively simple matter. take stock of the information which has been obtained by the work its application to shipbuilding and boiler-making purposes, As far as we know, the best attempt to express in this thus carried out. have been removed, steel is nevertheless but little used, in way the structural value of a given material was made by In considering the selection and arrangement of a set of electric this country at least, for bridge building, and hardly at all Mallet in his work on the “ Physical Conditions Involved light plant, the first question is the character or kind of light to be in the Construction of Artillery, ” and in a paper in the employed ; secondly, the kind of electrical generator most suited for roofs or buildings. In stating his case, which may be for this light ; thirdly, the kind of motive and means of transmis­ termed steel versus iron, Mr. Matheson resorted to a series “ Proceedings ” of the Institution of Civil Engineers, sion from motor to machine; and fourthly, the apparatus and of propositions as the most convenient way of eliciting vol. xviii., on “The Coefficients Te and Tr of Elasticity material for the distribution and regulation of the current between opinions on the various points. In these propositions he and of Rupture of Wrought Iron. ” These coefficients and at generator and lamp. If we speak, then, first of the kind of referred to the certainty with which plates and bars of all expressed the balance in any material between strength light, we have the choice of three kinds—that is to say, we have and toughness, or the work done by an extending the choice of lamps of three kinds, viz.: (1) the arc lamps, (2) in­ forms are now rolled of steel, to the advantages in candescent lamps, and (3) semi-incandescent lamps. respect of sizes and weights as compared with iron, to the and compressing force on an elastic body at the point Of these lamps, then, experience has only confirmed the reasons superior mechanical properties of steel, to the question of where its elasticity becomes permanently impaired and for selecting the arc lamp as the most suitable and the most manipulation, and to others leading to the proposition that at the further point where rupture occurs. These co ­ economical for lighting large spaces, and, except in ornamental structures of steel are superior to those of iron, but that efficients express the value, then, of a structural material design, little now remains to be done in the improvement of these by two indices, either for tensile or for compressive resist­ lamps other than in the perfection of the carbons employed. The there are at present only a few instances in which its use incandescent lamp will probably remain permanently the best for offers any pecuniary advantages, and that this limit to ance, and the formulae at length and their application will general indoor illumination. It may, however, be found upon the application of steel is partly due to official rules and be found illustrated in The Engineer for the 8th October, further experience that it is adapted also for street lighting pur­ PE poses. Two things remain to be done in its improvement, viz., partly to exigencies of design. On all these propositions, 1880. In the simplest form the formula for Te = —and and more, Mr. Matheson enlarged, especially from the the perfection of the carbons and contacts, and the mechanical arrangements in processes of manufacture, so as to produce it at a point of view of the constructor ; but the question to which FE' . minimum cost. The experience gained with these lamps is at particular attention is drawn is what is considered the T r — 0 , in which P and P' = elastic and ultimate 2 ’ present too small to speak with certainty as to their length of anomalous character of the rules of the four official bodies life. Many of the earlier ones lasted for comparatively a few by whom the maximum strain which shall be visited on strength respectively in pounds, and E and E' the elastic and hours, but improvement has been so rapid that the later ones the parts of structures is limited, and the properties of the ultimate extension respectively in feet. Thus for a material have an average life of from 700 to 1000 hours. Some of the material determined by certain stipulations. Of these we have having an elastic strength of 15 "3 tons, an elastic extension makers guarantee this, others do so by guaranteeing renewals of 0"0143in. in a length of 1ft., an ultimate strength of over and above a certain percentage, on condition, of course, that first, the Admiralty demanding that the steel used for the lamps are worked properly. There is no reason why, as ships shall have an ultimate tensile strength of not less 24"06 tons, and total extension of 2-216in. in a foot, experience is gained in the manufacture of these lamps, and with than 26 tons, or more than 30 tons per square inch, with Te = 20-579, and the current regulated in accordance to the design of the lamp, the an elongation of 20 per cent, in 8in. By Lloyd ’s rules the _ Tr = 4978T ; average light of a lamp should be limited to a thousand hours. It minimum and maximum ultimate tensile strengths are While for a material having an elastic strength = 14'22 is quite probable that the average life may be extended to two 1 ton higher, or 27 and 31 tons, the other figures remaining tons, elastic extension = 0"0288in., ultimate strength = thousand hours, or to even a longer period, and that the first cost 42'3 tons, and total extension = 0"67in. in one foot, of a lamp will be far less than the most sanguine expect now. the same; while the Liverpool Underwriters raise the It is, perhaps, unnecessary to say more of the semi-incandescent limit to from 28 to 32 tons. The French Admiralty rules Te = 38 "22 and lamp than that it does not seem to command itself to public favour, demand higher minimum strength than any of the English Tr = 2693 although the light is steady and of moderate intensity. This may bodies, and they apportion the strength according to the To arrive at the values of the same materials for work be on account of its cost. Referring now to the electric generator, section. Thus it is only when fin. in thickness is reached under compression it is only necessary to substitute the which is usually a dynamo-electric machine, a selection has to be compression for the tensile strengths, and the corresponding made in accordance with the selection which has been made from that the minimum of 28 tons is allowed, no maximum among the different lamps. It is, therefore, necessary to take into con ­ being prescribed ; but the minimum increases inversely elastic and total compressions. From the preceding examples sideration the special requirements of the chosen lamp—for example, as the thickness of the plates or bars, and for plates relating to tensile strains and extensions it will be seen arc lamps may be chosen, such as Crompton ’s, as used at King ’s above |-in. and under fin. is 28j tons. The rules of that the comparative structural values of very diverse or Cross, which require for three lights in series a machine having an the fourth English body referred to —namely, those of the similar materials may be gathered at a glance, which is E.M.F. of 250 volts, or we may have incandescent lamps such as not at all so easy to do from an inspection of the four sets the 20-candle power lamp of Swan, which require a machine Board of Trade—are of a different character, and prescribe having an B.M.F. of 45 volts, or the Edison 16-candle power lamps the maximum working strains to which steel shall be of figures showing their mechanical properties in the four which require a machine having an E.M.F. of 110 volts, or subjected in structures, and not the quality, as is done by separate elements. It will, however, be seen that to com ­ 8-candle Edison which requires but half that E.M.F., and so on. It the other bodies. Not only is the quality of the steel plete the index it is necessary to take Te and Tr simul­ will thus be seen that the E.M.F. of the machine must be propor ­ prescribed by the Admiralty and Lloyd ’s, but the strains taneously into consideration, for although Te is the tional to the resistance of the lamps. If this is not insured, and if coefficient of the greater importance in most cases, the machine has too high or too low an E.M.F., the carbons in the to which it shall be put are as far as possible limited, one case will be destroyed, and in the other will not be heated to because the thickness of plates and framing and character Tr becomes a very important element in the mem­ incandescence. From an engineering point of view we may compare of rivetting for ships of given sizes and for given pur­ bers of, say, lattice structures, where ductility may theE.M.F. to head or to pressure of water in an hydraulic system, and poses are also prescribed. The Board of Trade rules, be usefully brought into play before every part of the thus engineers not conversant with these terms will understand that however, secure no control or check upon the quality structure assumes the strain or work assigned to it, either E.M.F. is the electric condition necessary to overcome the resist­ ance of the lamps, just as a given pressure in the hydraulic system or character of the steel employed, but only enforce so through reasons of a practical character, or as in continuous girders through slight unassignable changes in the is necessary to cause the plunger of a hydraulic press to move ; low a maximum strain that a structure cannot be other and engineers will also understand that a quantity of water under than safe. This maximum tensile strain is tons. It is strains. There would be little difficulty in combining that pressure, or similarly of electricity under that condition, is urged by Mr. Matheson and by others that this very low these indices, though there are several questions to be necessary to keep the plunger moving. Thus a certain E.M.F. is maximum is obstructive to the progress which should now considered in making the best combination, and the sub­ wanted to overcome, so to speak, the resistance, and to put the be made in the application of steel to structural purposes, ject is one which might be usefully dealt with by our lamp in a certain condition, as well as a certain quantity of elec­ readers in our correspondence columns. tricity to keep up the condition. and that the system of low minimum and maximum tensile The unit of E.M.F. practically employed is termed a volt, which strength imposed by the Admiralty and Lloyd ’s acts in the Before concluding, it should be mentioned that though is approximately the E.M.F. of a standard cell. This cell gives a same way, and it is moreover pointed out by them that the Board of Trade rule fixes the working tensile load at certain deflection on, say, a tangent galvanometer, and the E.M.F. though so particular about steel, these bodies do not 6 "5 tons per square inch for steel in bridges, it was recom ­ of other batteries or electric generators is calculated in terms of trouble themselves one bit as to what sort of iron is put mended by the committee appointed to consider the sub­ such deflection. • ject, that a higher or other load might be allowed under Resistances are measured in terms of a unit called the Ohm. A into ships. copy of the ohm can, like a pressure gauge or 2-foot rule, be pur­ There is probably little doubt that mild steel having an circumstances justifying a change. This recommendation chased at any electrical instrument maker’s. As an illustration of ultimate tensile strength of 35 tons per square inch may was adopted by the Board of Trade, and Mr. B. Baker the ohm it may be mentioned that it is the resistance which an now be obtained with certainty and uniformity. It cannot, stated on Tuesday evening that it has been acted upon electrical current experiences in passing through a copper wire however, be said that this has been the case for a sufficient with reference to the proposed Forth Bridge, and a higher 10'29ft. long and 10 mils, in diameter. load than 6-5 tons allowed. If, however, it can be One of the most important points with which an electrical engi ­ length of time to have warranted any very material neer should be acquainted is that of resistances. An electric change in the rules enforced by the bodies referred to. Steel shown that uniformly homogeneous steel, having the current passes only when the conductor of that current forms a for structural purposes may certainly now be obtained necessary mechanical properties, can be always obtained complete circuit. This circuit in electric light arrangements is which may be trusted to do the work to which it is put, with certainty, it is undesirable that every engineer who divided into two parts, one being part of the machine itself, the but this trustworthiness is obtained by making steel of has a bridge on hand should have to make a special other being the cables and lamps, &c., external to the machine. representation to the Board of Trade for permission to use The internal part of the circuit is the wire and commutator con­ such great ductility and malleability that it will stand almost nection from brush to brush, and the resistance of this would any manipulation, and then using it with a factor of safety steel as steel, and not as something only a little stronger remain constant after the machine is made if it were not that the so high that a comparatively low limit of elastic strength meets than iron. Mr. Baker also stated on Tuesday that an metal varied its resistance -with change of temperature, increasing the requirements. In most, though not in all cases where this extensive series of experiments would be made on the with increase of temperature according to the following rate, ascer* mild steel may be used under tension, it is a matter of qualities and properties of steel, in view of the construction tained by Dr. C. W. piemens small importance that its elastic strength is not so great as of the Forth Bridge, and we may express the hope that r — a Ty + 3 T ± 7 in the harder steels, or that it does not exceed about, say, the elastic extension will not be omitted from these observa ­ T = absolute temp, reckoned from — 273 deg. C 55 per cent, its ultimate strength, for in no case would it tions, as it too commonly is. «, 3, y — constants thus for be knowingly strained to its limit of elasticity, as the usual Pt r = 0-039369 T^ + 0'00216407 T - 0'2413 factors of safety would prevent this. SOCIETY OF ENGINEERS. Cur = 0-026577 T* + 0‘0031443 T - 0-22751 Mr. Matheson proposes that the Board of Trade rule Fe r = 0-072545 T* + 0-0038133 T - 1'23971 should be changed to 8 tons per square inch as the maxi­ NOTES ON ELECTRIC LIGHT ENGINEERING. The heating of the armature coils is sometimes looked upon as mum tensile strain instead of tons, and it is clear that affecting the insulation only, but it will be seen that the heating On Monday evening, the 6th inst., the following paper, by Mr. until some such change is made very much progress camiot also affects the resistance of the circuit, and hence the steadiness be made in the employment of steel for bridges and roofs. C. H. W. Biggs and Mr. W. Worby Beaumont, was read before of the light. The external part of the circuit is the one coming The advantages which would attend the use of steel in the Society of Engineers :— under the control of the engineer after the machine is made, and Twenty-five years ago the man of science was able to obtain a it is here that his knowledge of the laws affecting resistance can these structures are very great, as the principal load of very fair idea of the position of knowledge of a number of branches be applied in the successful arrangement of any system of electric large bridges, namely, the dead load, would be immedi­ of science ; but since then, so great has been the progress and the lighting plant. Generally, then, every substance resists more or ately enormously reduced, and especially would this be the development of each branch that it is as much as he can do to less the passage of electric currents, some, such as metals, resist the case if the harder steels could be used for compression make himself thoroughly conversant with any one branch. Just passage but slightly, and are called good conductors ; others, such members, advantage being taken of their very high elastic so is it with the work of engineers. Gradually but surely an as glass, shellac, gutta-percha, &c., offer an enormously great engineer has to confine himself and his labours to one distinct resistance, and hence are called non-conductors or insulators. The strength under strain. It would enable us to build branch—it may be sanitary engineering, or bridge work, railway fundamental formula, which should be the basis upon which the large bridges of superior type and appearance at a low work, and so on —whilst quite recently the extraordinary develop ­ calculations are made, is called Ohm’s law, viz. cost. Before, however, any united action is taken in ment of the applications of electricity seems to point to a great order to obtain some relaxation of the Board of Trade future for the engineer who devotes himself entirely to this branch C = rules, it seems desirable that measures should be taken to of work. Although electrical engineering is still in its infancy, sufficient has been done to show the necessity for a better compre­ in which C=current arrive at a definite index expressive of the structural value hension of the laws which determine the success of the work E=electromotive force of steel employed under tensile and compressive strains. undertaken. Accidents at various places have, as is well known, R=total resistance. At present the value of a material is expressed by four occurred ; but each of these accidents can be traced to ignorance, We have previously pointed out that the total resistance is or a culpable negligence of the laws just referred to. There is no d