A j4afier /rese-ned al Ye Eleven z General M-ee- og, of tIe America Insiatule of Elecl icza En- gineers, Philadedfiha, May s66t, 1894. Fresi- det osionsnin idie ChaIr.

UNTNIPOLAR DYNAMOS FOR ELECTRIC LIGHT AND POWER.

BY PROF. F. B. CROCKER AND C. H. PARMLY.

INTRODUCTION. The object of the presen-t paper is to cal1 attention to the fact that unipolar dynamos and motors are mueh imore practical and generally applicable than is ordinarily supposed to be the case. The termii unipolar dynamo is here used in its ordinary sense to. designiate a machine in whichl electric currents are genierated by the eonti,nuowus ettivn of lines of force. These maehines are identical in principle witlh the original disk machine of Faraday. The term nuiipolar is by no means satisfactory, but is almost uni versally used in connection with machines in which the magniet- isma in the armatuLre is olot reversed by the rotation of the latter. In this broad sense, however, it is also used to inelude maclhinaes of the Mordey an:d other types, in which the lines of force always, pass through the armuature in the same direction; but in these mnachines there are separate pole-pieces and the linies of force vary in the armatnLre just the same as in the ordinary bipolar and multipolar types. Moreover, machines of this kind usually have their armatures wound withi many turns of wire, they re- quire a in order to generate direct currenlts, and are radically different in principle fromn the continuous ctutting ma- chines which are the subject of this paper.' The term " non-

1. The term unipolar is also applied to the Ball dynamo, but this also acts like an ordinary bipolar machine, except that the magnetic circuit is completed through the air and it is no sense a ' continuious cutting " machine. 406 1894.] CROCKER AND PARMLY ON UNIPOLA DYNA Os. 407 polar " suggested by Forbes is preferable to uniipolar, but it is rather mneaningless and is liable to be understood to mriean any kind of iron clad m-achine. Unipolar dynanmos aIe also called disk or tube msachiines since the is usually made in one or the other of these forms. Neither of these terims, however, covers the other, and they are liable to be conifused with other types of machine similar in form but different in principle. Th-ie writers of this paper suggest continuous _pole dynamo as a good name for this type of generator. HISTORICAL NOTES. The first nmachbine of the unipolar type was Barlow's Wlheel' described by him in- 1823, anid consisting of a star-shiaped disk with long points revolving between the poles of a . A currenit was passed through the particular portions of the disk between the poles of tlhe nmagnet, and the disk was tlhereby caused to rotate. T'his apparatus was, therefore, the first unipolar motor. Faraday's disk machine, constrnLcted and described by him in 1831 was the prototype of the dynamon and was also a unipolar' muachine.2 He also miade an apparatus working upon the samue princyiple buit consisting of a copper cylinder suspended over, and revolving around the pole of a bar magnet. This was the first form of tube dynaimo.3 Faraday even at that early time appre- ciated tlhe difficulty of obtaining a sufficiently high E. M. F. froim genierators of this type, and aitternpted to attain it by lhaving- several disks placed side by side and revolving in opposite direc- tions. Simuilar ideas have beeni re-invented time after timne, and eveni at present it is a favorite field for invention For nmany years after Fhaday's discoveries the uni polar machine was almuost neglected, progress being in the direction of iriachines lhaving many turns of wire in the armature to obtaini a high E. M. F. In 1878 Siemens constructed a unipolar m-achine with a tuLbe aria- ture of considerable size for actual coinmnercial use in electro- metallurgy. Delafield shoiwed at the Philadelplhia Electrical Ex- hibition of 1884, a un:iipolar c ynano of the tuibe type which generated a current of few volts and a large numnber of amperes. Forbes has also constructed aind described unaipolar machines, in wlhieh the armnatture is in the form of a cylinder of ironi revolv- ing within a field magnet which completely surrounds it. The 2. Experimental Researchex, vol. i., art. 85. X. Loc. Cit., art. 2 9., 408 (IROCKERAND PARlL 0 N UNIFOLARDYNAMOS. [MayNI mechanical and magnetic design of this machine is very ingen- ious, and from the practical staindpoint it was a considerable irn- provenient upon anytlhing which preceded it. The highest E M. F. obtained from these miiachines, appears to have been about 6 volts, and nothing f-Lrther lhas beeni hear-d concerning them for several years. The complete n-eglect into which this type of dynam) has now fallen, is best proved by the fact that no example of them was shown at the Chicago Exposition. Ini fact it miglht be said that their present imnportance is actually nega- tive, sinice the only attention which has beeni given to them lately 0consists of a few articles oni "UUnipolar dynamos which do not work," that have appeared in the electrical journals durinig tlhe past few montlhs. GENERAL PRINCIPLES. The action of unipolar dvnamos, is based upon the fact that a ',onductor mooving in a magnetic field so as to cut lines of force will lhave an E. M. F. set up in it whether the field be uniform in inte'n-ity or variable. The error is very comnmonlly made of sup- p9sing a varibiSon in the number or density of thie lines of force is n-ecessary in order to produce a cnirrent by magneto-electric induction. As a matter of fact, however, an E. i. F. must always 'be produced if any lines of force whatever are cut. It may happen, and in the case of a coil of wire it ustially does hlappen, that lines of force are cUt in one direction by one portion of the conduietor, and in the opposite direction by the other portion, in wlhich case one effect lneutralizes the other. The simplest ex- ample of this is a closed metallic ring moving perpendicularly to the lines of force in a uniform field. In this case one-half of tlhe ring generates an 'E M. F. in one direction an-d the other half in the opposite direction, so that no current is produced; but the fuill difference of potential corresponding to the number of lines cut per second, will nevertheless exist' between tlhe two sides of the ring. In the unipolar dynaino or mnotor, a variationi in the lines of force is not onily unnecessary but is positively objectionable, silee it would cause serious losses from eddy currents and hyster- esis, whereas if the field is perfectly uniform these losses are practically avoided. Any break or weakening of the intensity of the field due to 4l)low -holes " in the casting, for example, would allow the arma- 1894.1 CROCKER AiD PARlI[LY ON UNIPOLA.R DYNAMOS. 409 ture current to flow back at that point, aud would act as a sort of short-cireuit on the rest of the armuature. For example, in Fig. 1, le. A A represent a disk armature revolving upon a shaft s. The field is of niuifo m intensity anrd an equal E. M. F. iS gene- rated at each radius, as indicated bv the arrows, except that the portioin B of the armature is in a weaker field. In this case the current will flow back through the portion B, as indicated by the arrow, and the arrnature will be like a sbort-eircuited one of the ordinarv kind. This back current will flow and produce lheating and loss of enlergy, even though the external circuit be open.

A/

X /~~~~A

FIG. 1.

It is also a commion error in regard to unipolar mnachines, to 'suppose that the armature cannot be made of iron. As a nmatter of fact, iron or steel is usually better than copper because it is a much better conductor of magnetism, and the reluctance of the magnetic circuit cean be reduced far below what is possible with a copper armature, the onily air-gaps beinog the small imechanical clearances required for free rotation. A steel or wrought-iron armature is muich stronger than one of copper, not only on account of its greater strength for the same thickness, but also because it can be made much thicker without causi'ng any appre- ciable increase in the reluctance of the magnetic circuit, whereas a copper armature would have to be made very thin. The greater specific resistance of iron is not objectionable, siniee an 410 CROCKFE AND PARMLY ONlNIPOLAR D )Nf tMOL. [May 16, armature comiposed of it, would have a current capacity far greater than is re(quired, and the thickness cani be increased to make up for the higher specific resistance.

METHODS OF MULTIPLYING THE E M. F. The plani suggested by Faraday, was to haive two or miore disks side by side revolving in opposite directions, the ediges being conniected together by mercury or other contacts so that the eurrenit flows outward from the center to the periphery in, one disk, inward in the next disk, and so on in series. In this way it is possible to imultiply the voltage as imany tiines as there are disks, but there are obvious mechanical difficulties in rotating- these disks in opposite directions, and the numerous electrieal contacts would be decidedly objectionable. Hundreds of modifi- cationis of this idea lhave been suggested; for example, Siemens in the tube maehine already referred to, multiplied the E. Mi F. by splitting the tuLbe longituLdinally and placing rings on the end of the tube, so that the current was carried first tlhrough onDe SeCtiOn of the tube, thent through another, and so on. In this way the circuit may be threaded through the magnetic field a number of times, each timiie a certain E. M. F. being added. Another plan to accomplish the same result would be to have a niinmber of thin tubes placed concentrically otne within the other, but separated by a layer of insulating mnaterial, and connected so tlhat the cur- rent passes first through one tLibe, is then carried around outside of the machine and is passed through another tube, and so on throughi all of them in series. There is no theoretical limit to inereasiung the voltage by somiie sueh lethod, but practically any of these constructionis would lhave very serious, if niot fatal, mechanical and electrical difficul- ties. It would seem to be far preferable to eoistr:uet the ma- chine of sufficient size and run it at sufficient velocity to get the required voltage without complicating the construction. There is no objection, lhowever, to connecting two unipolar maehines inl series, both being driveni, for example, by one engine, which niay be belted or direct coupled, and there is no serioius complication involved in operating four or even more machines in series. The first cost and attendance required would probably be much less with fouir nipolar machines than with one machine of the ordinary type and of equivalent capacity. One very convenient way to multiply the voltage of a dyinaimo would be to charge a numiber of storage batteries by means of a 1894.] CROCKER AND PAR.MLY O U-NPOLAR DYlNAAI08 411- low voltage machinie of, say, 10 or 20 volts. When charged, these batteries could be connected in series to give any desired E. M. F., 115 or 230 volts for example, and the actual working current would be supplied by themn. The charging conild be done at houLrs when the current was not required, or two batteries could be used alternately to give an uninterrnpted supply of cUr- rent. In this way two advantages could be realized; first, the cheapness and simplicity of the unipolar dyniamo, and, second, the uniformity of load witlh which the enginie and dynamo could be ruin. PRACTICAL DESIGN. It is nlot difficult to get up complicated pnizzles in connection with peculiar forms of unipolar machines, but in the simple a-nd probably the only practical forms, we have a single straight coni- ductor which euts all the lines of the magnetic field once per revolntion. In the case of the disk maclhine we can consider any- radius of the disk as constituting the conductor, anid in the case ,of the tube iacbhinie it is aDy element of the cylinder. The theory of the imagnetic circuit is niow well understood, and mnoderni practice in the design of dynanmos and motors fully- appreciates thel advanitages vhiich result fromn making the ratio, between the length of the magnietic circuit anid its cr,oss-section a minimum. In a unipolar dynamo, this.reduction of the reluc- tance of the eirecuit can be carried farther than in any other class of maclhines, because the creation of an electromotive force by the continuouis cutting of the lines of force, enables a form of magnetic circuit to be employed, which in the higheest degree combines the advantages of miinimulm length with maximum cross-section. This type of magnetic circuit is the circu-lar ring for if we surround a circular coil D D of wire carrying a current, with two circular rings of semi-circular cross-section, as shown in Fig. 2, we shall form a closed magnetic eireuit of maximulm induction for minimum excitation. Cut this circuit alon, any radius 6f the circular cross-section, and a magnetic field is ob- tained in which, if an inductor be rotated about A x as an axis, there, will result a continuous cutting of the linles of force, which onily requires that appropriate collectors be provided to formn a. closed electrical circuit, in order to procure a current of electric-- ity. The magnetic circuit is indicated by dotted circles with arrowheads. 412 CROCRK RAND PARMLYON TNIPOLAR DYNA LObs. [May 16, There are, however, but two directions in which the miiagnietic ciricuit can be cut, so as to form a field suitable for practical- use. Ouie is along the radiuS C D, in which case the inductor will rotate in a plane perpendicular to the axis, and the other is along the radius E D, in whicl case the inductor will move in the con- vex surface of a cylii der whose axis is the axis of rotation. In the first case the armature of the resulting dynamo will be a disk, and in the second case a cylinder. In both, the way to collect the current would be to provide one set of collectors at tlhe edge of the ar mature, and thAe other set at tl-he axis, thus utilizing the shaft as a portion of the electrical eircuit. The rel-

F

A X i>

FIG. 2.

'ative advantaoes and disadvanitages of tlle d'isk and cylinsder formis,' depend ulpon circumstances in each parti;cular instance. In many cases tl-ie single iiiagnetic circuit, shown i-n Fig. 2, is ilot tlhe best for practical usex because it would be nlecessary to mak-e the diameter of thelin larger tllan it woiil bei w ings were placed side by side, as slhown in Fig. I3 and Fig. 7, ~sinee in this type of dynaino we niust depend wbolly upon the total Mmagnetic induction aiid thle speed of rotati-on? to -produce the elee- tr.omotilve force, as it does not'appear to be practi'cable to greatly, augimeint the voltage by mualtiplying the iiuinber of inlductors.- 1894.] CROCKER ASDP.ARMVLYONU JIPOLARDYNA O0. 413 Another conisideration of importance is the volume of the ring. It is evident that any required area of' pole surface can. be provided, by keeping the mean diamneter of the rinig constanit, and varyilg the radius of the circular cross-section between cer- taim limits, or by keeping this radius constant and varying the, nean dhlmeter of the whole rinig. It is also evident that for any particular required area of pole surface, any decrease in the mean diameter of the ring requires a corresponding increase in the radius of the circular cross-section and vice versa. In Fig. 4 let the circle whose radius is )v and the distance of whose center froin the axis of x is b, revolve about that axis generating a vol- ume of revoltution. This volume forms the field imagnet, and containis nearly all the weight and a large part of the cost of the

AA ___.X____

FIG. 3. mnaehine. This volume, which is an annulus, may be determined by the centrobaric miiethod, according to which a volume of rev- olution is equal to the generating area multiplied by the eircunm- fereniee described by its center of gravity. Hence the volume V = 2ri 2vb (1>! The factors r and b must have certain values in order to pro-- duce the required E. Al. F., the latter being usually the definite object for which a dynlamo is designed. The values of r and b, are founid as follows: Let E the requtired electromnotive force expressed irn volts. Let n -the number of revolutions per iminute. Let B = the number of inagnetic lines per square inch. Let A = the number of square inches of pole surface. 414 CROCKER AND PARMLYON UNIP'OLAR DYNAIOS. IMay 16,

Let r the radius of the circular cross-section of the rLng ,expressed in inclhes Then because the two rings are wounld so that the electromo- tive force due to the field of one is added to that due to the other, each ring must be designed to provide eTnough lines to F F generate only - 2~~~~~~~~volts. This requires that - X 10" lines muIst E n be cut per second, or X 10' ± per revolution.IHence the number of square inelhes of pole service needed is A 3 Ex nB O(2) As shown in Fig. 3 the core of the ring will be utilized for the exciting coil, and for simplicity of construLction we shall make its

Y

(b X

FIG. 4. cross-section square. It is desirable to keep the size of this square as small as possible, because any increase in the length of a side increases the diameter of the circular cross-section an equal amnount, and, therefore, enormously iniereases the volume. R-ut this square cannot in practice be made indefinitely simall, because too great a concentration of the lines immediately surrounding the coil nmust be avoided, and space enough provided to give ample excitation. One-half of the radius appears to be a length suitable for the side of this square. Referring to Fig. 5 we shall then have an area of pole surface. A = 27T b Ii . (3) But we have seen in equation (2) that we need an area equal to 1894j] CROCKEJR AND P'ARALY ON UNIPOLAR DYNAMOS. 415 ,30 F x l08 n_B hence equating this withl the above value of A we have 24rsr,30EX lO

The ratio between Z and r which seems to best fulfil the con- ditions of good design is b-2.5 r. Making this substitution above and designrinig so that 2 b equals tlle outer diarneter P of the armature, we have i?~=nV2 EB' (4) whiclh shows that the dimensions of the maclhine are directly proportional to the square root of the required electromotive force, and iuversely proportional to the squiare roots of the speed and of the mnagnetic induction. It is to be further observed that sinee for machines varyinig widely in their outputs B is nearly constaint, the most important ratio in equation (4) is E In the design of a mnachinie for any special voltage, however, E is fxed, and, therefore, the only arbitrary quantity is n; so that the whole problem is to determine the valnes of D and n whiel give the b)est design. Let v the peripheral speed in feet per second, then we have zn D 720 wheinee O A 720v () Substituting in this equation the valu-e of D given by eqtuation (4) we have n = 0.00000825 , (R) and P _10 PI 3DB9/ (7) from which the values of P and n can be inmmediately deter- mined for any preseribed values of E, B, and v. The maximnum velocity in feet per second, which the rimrl of a fly wheel may vith safety attain, is given by the eq-uatiolnl V 3 /t 1. Thurstoni. Manaal of thje Steam Engine, Part ii., Art. 3, p. 422. 416 CROUI(h? ASD PARIJILY ON UNIPOLAR DYNA lOS. [May 16, whlere t is the safe stress of the metal in pounds per square inicli. If the inductor be of wrought iron or steel for whichl t = 10,000, we shall have v = 300; and if of east ironi for which t - 5000 we slhall have v = 210 nearlv. The much higher perineability of cast steel over cast iron, and its coinparat;ively small additional cost m-lake it the metal most economical to use, and we .l:hall, tlh'erefore, assume that the inductor is either of virought iron or of cast or forged steel. Hence makinig v - 300 in equation (5), we fnd that the maximumu value which the prioduct D n can attain is D n -68,750. This then is a condition whlich must not be exceeeded in order that the design shall be meehaniclleay cafe. Making iv 300 in equations (6) and (7) we have - .7425 E (-s

b A X.

FIG. 5. and 108IO7EB' (9) If the armnature were mlade of fo'rged steel it would be allowable to assume t to be as high as 20,000 siince the armnature is ent'irely enelosed anid couLld do no lharm eveni if it buirst. In this case v - 423. We have now obtained data for finding the niuLmber of ampere turns required to produLce the necessary excitation. As a first approximation, and to enable us to determinie the general condi- tions, the cross-section of the rinrg was assumed circular In practice, however, this fornm of section must be modified, for a reference to Fig. 2 shlows that in order to give the samne area of cross-sectiont perpendicuular to the direction of the lines of force at D c and D F as at D E we must cause the circular cross-section 1894.] CROCGKERANDPARJILYOXNUNIPOLAR 9YNA OS. 417 to bulge out on the side toward the axis and contract on the side away from the axis. OnI account of the dinlinution of cross-section due to the air- gap and to the holes for the brushes, it is found necessary, how- ever, to preserve for the outer portion of the ring a semi-circular cross-section, and it ultimately assumes the form shown in Fig 6. The meain length of the magnetic eireciit is, therefore, the length of the senmi-circle a b c plus the length of the curve a d c. Assuming the total air-gap to be .06 /1D we have the data for determining the arnpere-turns required. The amount of eurrent which the dynamo can generate is de- teri-nined by the carrying capacity of the armature and bLrushes. F

/ b \

FIc. 6. Sinee .2 j/ D mnay be assumned to be the thickness of the induc- tor, we lhave for the cuirrent-carrying cross-section of the arna- 4-uure .2 72 square inches. Since iron or steel would carry at least 200 am- peres per square incli the current capacity of the armature is C= 125.D1 (10) which at E volts gives the capacity of the machline in watts W1 = 125.7 EDw (11) To show the application of these formuloe to two widely dif- ferenit classes of machines, let it be required: 418 CROCKhR AND PARMLY ON UNIPOLAJI DYNAMOS. [May 16, First. To design a belt connected dynamo to generate 10 volts at 1,200 revolutions per minute for electro-metallurgical pur- poses. Making field a-nd armature all of cast steel, anid taking the induction B at 90,000 lines per square inch we have from equation (4) D _ 24.28 = diameter armature in inches. Second. To design a direct conneeted dynanmo to generate 130 volts at 200 revoluitions per minute for light and power puirposes. As before we lhave D 214.4. These figures show the immense possibilities of the unipolar dynamo, possibilities so immense indeed that the practical diffi-

FIG. 7. culty is not in the designi of the dynamno, hut in the design of the steam-l enigine. Two m-achines liike the last, coupled at tlle ends of the shaft of a lo0,000 ii. P. engine would suipply iore current than is at present consumed in any two cities in the world. Nor is the excessive current a disadvantage, for while we wouLld point out the tremendous capacity of one of these dy- namos, there is no niecessity for rnnning theIn at their maximum output. Even if we diminiished the current to one-liundredth its ultimnate value, and only use a 1.000 B. P. enigine to drive the dynamo, the loss due to tlhe resistance of the arlmtatuire will still be only four or five per cent. 1894.1 (GROCKER AND PAR LYON UNIlPOLAR DYNAMOS. 419 A more economical way of supplyiing current for liglht and power, however, wouLld be to design unipolar dynamos of less curlrent capacity and less voltage, and then jo'in in series on the sami;e shaft as many as are needed to produce the required elee- troinotivre force. For example, we may generate 130 volts by joining in series on the samue belt-driven shaft two dynamos each giving 65 volts at 800 revolutions per nlinute. The dimensions will be as follows: D= 75.81. IRunning this combination at one-tenth of its ultimate capacity by means of a 1,250 H. P. engine, we should still have a very high efficiency. Xet/ods of Tainy Qfl the CLAt ent.-The best mneans of naking electrical cornnection with the revolvirng armature is a very imnportant but somxlewhat difficult matter. A great many devices have been tried or suggested. The principal of these are brushes of copper gauze or carbon, and mereury contacts applied to the edges of the dislk or tube. Belts or stiraps made of fex- ible sheet copper or copper wire cable have also been used. One application of this latter device consists in connecting together two armatures by such a belt whereby the E. M. F. of the two miiachines are added together, and, if necessary, the mnechanical power for driving one or- both machines can be transmitted by this same belt. There woilid not seem to be any great difficulty in applyinig brushes to a unipolar dynamo; in fact for a given currenit it would seein to be a nueh easier problemn thanl with a direct current maclhine haviing a comm-utator, since the former lhas a continuous and smooth bearing surface for the brushes in- stead of the somewhat uneven surface of copper and mica which exists on a commutator. It is a fact tllat the speed may be very high, but witlh the perfectly smooth surface anid by the use of bruishes with light pressure, and comIposed wholly or partly of graphite or some friction nmetal which might even be lubricated, it would appear to be possible to take off a current sufficient for almnost any electric light or power use. If the very high speed makes the friction too great to permit of the use of brushes, the edcge of the tube or disk may be airanged to run ini a mercury troug,n, thereby making a good electrical conitact with very little friction. In this case, lhowever, prov;ision should be made to prevent the mereury, from being thrown off by centrifugal force, 420 CROCKER AND PARMLY ON UNIPOLAR DYNAMOS. [May 16, as by the use of a guard ring extending all around the edge of the tube or disk. Another device would consist of ball bearings applied to the shaft or to the periplhery of the disk or cylinder. These might perform their ordinary mechanical fuinction and also serve to take off the current. _7Method of Drivin.g-There are various meehanical arrange- ments available for connecting a iunipolar dynamo to the soilree of power. The simplest would be to connect the dynamo to the engine by means of ordinary leather belting, either with or with- out the use of a countershaft. In this way a hiigh speed can easily be obtained if desired, anid is perfectly permissible since the solid disk or tube of steel wlhich constitutes the armnature is capable of a muluclh higher speed of rotation than the ordinary built up form of armature now uised. It would probably be safe to have a solid steel armature run at two to four times the speed of the ordinary armature having the same diameter. If the dynamo be directly coupled to the engine, the speed of the latter and the diameter of the armature may be most econom- ically proportioned to give any desired voltage by the application of the previous equations. An arrangement which seems to possess great advantages would consist of a stearu turbine ruinning at 10,000 or 20,000 rev- olutions per minute, which is the ordinary speed of such ma- chines, directly couipled to a unipolar dynamo. These two machines seem to be admirably suited to each other, the very high speed of the turbine compensating for the fact that there is, but one inductor. This combination entirely avoids the necessity for reducing the speed of the engine by gearing which is done in the case of the Laval turbine, or the risk which is involved in running a wire-wound armature and sectional commutator at z speed of 10,000 or more revolutions per minuite, which is the plan adopted with the Parsons turbine. Advantages of Unipolar Dynamos and Xotors.-The great- est advantage of the unipolar machine is its extreme simplicity. Its armature consists of niothing but a solid cylinder or disk of steel, or other suitable metal firmly mounited upon a shaft. We have only to compare this construction with that of the ordinary armature consisting of hundreds of pieces of sheet iron bolte(d or otherwise held together to form the armature core, and wouniid with a great many turns of wire or bars of copper which have to be thorotuglhly insulated from each other, and which are not very 1894.] CROCKERANDPAR LYONUNIPOLARDYNAMOS. 421 securelv held in place. In addition to this we have the commnu- tator, conisisting of fifty or more sections of copper separated by strips of mica and held together by ntuts, etc. The electrical connections between the armature and commutator also add to the complication. In short, it would be difficult to finid any two pieces of machinery in which tlie contrast between siinplicity and complication is greater. The constrtuction of the field mnagnet and the rest of the machine is also very simnple. The elimina- tion of the commutator, although already mentioned in connection ivith simplicity, is ineverthieless an essential feature of this type of machine and is a great advantage. The almost infinitesimal armature resistarnce of these machines is decidedly advantageous, not only in increasing efficiency and decreasinig beating, but also because it causes the machine to regulate more closely either as a ,dynamo or as a motor. According to all accepted thleories there would be no hlysteresis in these mriachines, becatLse both the arma- 'ture and field are always inagnetized in exactly the same direction eard to exactly the samne intensity.' For similar reasons there would be no foucault currents, since the E M. F. generated in any ,element of the armnature would be exactly equial to that getierated in any other eletnent, and there could be no tendency to produce eddying currents. This perfect uniformity of the mag-netic field is secured by the construction whicil should be exactly symmetri- cal, the air-gap being preciselv tlle same at all points. The question of armature reaction is somnewhat doubtful since some authorities state that it is quite considerable, but in the (opinlion of the authors it is very small and certainly no greater than in otlher types of machiine. The arnature consists of only .a single turn, conseqtuently the maximum magnetizing effect of -tlie armature in amnpere-turn s is numerically equal to its current capacityT, and since tlle ampere-turns on the field would be made °considerably greater than this, the armature reactioii cannot be great. It is interesting to consider how armature reaction can occur in such a machine. The probability is that it has the effect of curving and slightly lengtheiuing the lines of force so that tlhey do naot pass perpendicularly froim one pole surface to the other in the air-gap, and have a spiral path in the iron since the Held current tends to produce lines of force in planes passing tlhrotugh the axis, and the armature current acts at riglht angles, 1. There might be sonme molecular friction due to cuttinlg the lines of force, ibut this would probably be slight. 422 CROCKER AND PARMLY ON UNJPOLAR DYNAMOS. IlMay 10, producing an inelined resultant. There can, of course, be no change of distribution of magnetism as a result of armature re- actioii, wlicll is the really objectionable effect that it produces in the present types of inachines, and in the unipolar maehines there are no back ampere-turns, and nio magnetic leakage. In conclusion we may say that unipolar machines are practi- cally indestructible, since they are so simple and so strong that they ar-e not likely to be darnaged mechanlically, and it is almost imipossible to conceive of one being burnt out or oblierwise in- jured electrically, since the engine would be stalled by the enor- mnous current before the armature could be fused by it. A machine possessing all these important advantages certainly de- serves a proiminent place in electrical engineering, whereas it now has practically no existence whatever. 1894.] DISCUSSION. 423 DiscusSION MR. CARL HERING:-Prof. Crocker hlas worked out the slhape of the field theoretically, but in practice I tlhink it could be clhanged to advantage by making the polar suirfaces at air-ga,p a little larger than he suggested, because by far the largest part of the reluctance of the magnetic field lies in the air-gap, even if an iron armnatuire is u1sed. Prof. Crocker speaks of the reaction of the armnature as not interfering appreciably witlh the field; in this I thiink he is no doubt correct, as the reaction of the arm-atLure will simnply shift the lines of force and perhaps make the magnetic circulits longer, but it Tiill not inake the field iLnuniform. But it is different with the currents from the bruslhes to the outside of the mactiine; unless these currenits are led otut perfeetly evenly tlhrough a disk, they will produce anl irregularity in thle field which will cause foucault currents in the armature, as Prof. Crocker stated in the beginning of his paper. I tllink one of the difficulties in the working of sulch inachines lies in this irregular slhifting of the fields due to the currents in the leading-out lines. About ten years ago I suggested that this nmight be partially overcome by leadinig thle wires out in the form of a spiral in the plane of the exciting coil, making it a sort of continuation of the , but I hardly think that it wouild be practicable. Prof. Crocker hias not made any allowance in his theoretical design for the open space which m-ust be left near the coil space for the brushes. The brushes for such a machine must be quite large and niitnerous, and there must be quite a large space left for them next to the coil space; this in turtin will change the proportioning of the field. Another difficulty lies in the fact that those br slhes must necessarily be in the i-nside of the field where they are not easily accessible. The inaclhine must, therefore, be designed so that it can easily be opened at this part, so as to get at those bruslhes without requiring too mnuch skilled labor and too many special tools; this will affect the simplicity of the -machbine somewhat. Regarding the term unipolar, I think myself that it is a very unfortunate one, but there is one way of looking at it which perhaps partially justifies its use. We can imagine unipolar as applying to the currents in the armature in wlich case it means a current which is always in onie. direction as distiniguished from one which is alternately in two opposite directions, as in the ordinary dynamos. This may serve at least as an excuse for the word, even if it does not jtustify its use. Regarding the maximum speed that Prof. Crocker has calculated, I would like to ask whether that is with, or witlhout a factor of safety. If it is without a factor of safety, it seemus to me that the dimensions calculated in those exanmples would have to be altered quite materially, because irn a construetion of that sort the factor of safety is an exceedingly important quLantity, even 424 CROCKEIJR AND PARIU4LY ON UNIPOLAR DDYNAMOS. [May 16, if it is taken as low as ttree, which is rather low for engirneer- ing struetures of that kind, it will make quite a difference in the size of the machine. PROF. CROCKER:-I will answer AMr. H ering's remarks in order. In regard to the air-gap I have with ine a blue print of a design which shows the increase of the pole area as indicated in the sketch. A very small addition of mnaterial would greatly increase the pole surface. The weight of the machine would not be materially changed, and all the formulas and relations would not be substantially modified. Of course, in starting out, we must have a ground work which can be varied slightly, in order to fulfil practical requirements. As the clearance is very small and the magnetic circuit short, I expect to have sufficient mnagneto-motive force to carry the lines of force across that air-gap even at a high density. In regard to the holes for the brushes, and the disturb-

FIG. 8.-Cross-section of Field Magnet modified to give greater Pole Area. ing effect which the current would h-ave where it came out, that could be overcome by having it brought out at a number of points. If necessary, more space could be allowed for the brushes, which might consist of blocks of metal or carboni resting on the surface of the cylinder or disk, the current being brought out by strips of copper passing throngh loles in the inachine. (See Fig. 8.) This trouble could be overcome by having a copper band around the armature where the current is taken off, which would act as a sort of collector to bring the current to the con- tacts. Various schemes of that sort will suggest theinselves. Of course the field magnet has got to be split in order to get the coil in. It could not be made solid, and it would be desirable to have it split up more or less in order to facilitate handling, etc. In regard to the factor of safety: I stated in the paper that the safe allowable stress is assumed to be 10,000 pounds, that is 1894] DISUUSSIi0e. 425 derived fiom Thurston's work oni the Steam Engine. Of course steel will have a breaking strain far in excess of 10,0000 pounds, and even 20,000 pounds working stress allows a factor of safety of three or four, and considering that the armature is absolutely enclosed and could do no harm if it burst, I think that is suf- ficient. Of course, we could make this disk or cylinder of forged steel, if necessary. There is notlhing to prevent it, though it would increase the cost somnewbat. I think those are the prin- cipal points you'mentioned. In regard to the niane "1u-nipolar," I thinik we must accept it, wliether it is good or bad. MAR STANLEY:-Some tirmie ago I constructed a very simple maclhine wlhich perhaps might be of interest to the INSTITUTE; reversing the whole design tlhat Prof. Crocker lhas illuistrated, keeping the conductor still, and moving the magnet, getting rid sof the outside contact. The trouble that I found in makilng SUCh a machine was, that I could not work with anything like such peripheral speed as Prof. Crocker suggests; so I took a copper band, the section of which was, say this (making a sketch), anid constructed a machine very muclh like Mr. Mordey's . The copper band projecting above anid below the field was permanently connected to one terininal of the circuit. The -magnetizing coil was placed in the cavity here as in the Mordey macihine, anTd theni a smooth surface brush, running at a low speed, kept contact with the interior of the copper band, and the current from the interior edge of the band was drawn off through -the shaft of the maehine. In this way I could increase the size of the machine very greatly and keep the peripheral contact ;speed down. Now this is perhaps very well, but what is the use of it all ? Why do we want such a unipolar iaehine, wlhren we can take a copper band, as Ferranti has done, and siunply saw it up as Mr. Mordey has, and take both contacts outside, and so have no sliding contact whatever. In this way we get approx- imately the same oultput from the machine that Prof. Crocker points out, and have all the advantages of the unipolar design. I cannot see why we wanit to coonstruet unipolar machines because they are unipolar. In the Mordey machine there are no slidinrg contacts. The type of the m-achine is only limited by the capacity to carry the currents generated, and there is very little armature reaction}, and so it seems to me it would be going backwards to attempt to btuild a machine with a conitact speed of 500 feet a second. MR. KENNELLY:-Mr. Chairmani, the paper is very interesting, because it resuscitates a type of machine tllat was supposed to be as dead as the mummies of Egypt. Some of the difficulties which are mrientioned here, while they are no doubt considerable, may perlhaps be still furtlher reduced by proper metlhods of conistruc- tion and design. Mr. Stanley lhas brouglht up an argument agairnst resuscitating this maclhine, showing that so sinmple a de- parture fromn this particular type will give a high pressure alter- 426 CROCHER AND PARAIL Y ON UNIPOLAR DNA.4MOiS. [May 16 nating cutrrent generator, but that seems to mne to introduce an- otlher questio i, the advantages of alterniating versus continuous currents. Beciuse if you were to make a continuouis current dynamo of tlhe samne pressure. youi wotuld have to introduce the commutator aind its disadvantages of cost, maintenance, etc The paper deals with the continutios current generator and shows h-ow the commutator can be avoided by utilizing a high per- ipheral surface speed. So the whiole question at issue is, is it ad-- vantageous to do away with the, conmutator and replace it by an arinature of very simple construction, of large diamneter, perhaps with a highi peripheral speed of brushi contact. But there are one or twro difficulties mentioned here, and whiich have beeni raised in discuission, that seem to me not so great as they look at first sihlt. For example, I do not think that any danger nieed to be anticiipated fromi sucli a small trouble as a blow-hole. Those whio lave tried to produce eddy ciurrents in a revolving shieet by deliberately naking a very irregular field, heavily grooving it, or putting cavities in it, will realize hiow deep the cavities niust be to produce serious disturbanee of that eharacter, and the reluct- ance of the air-gap is usually so great that the difference of- densitv whiicli is produced by any local variation on the surface is not large, and still less where the cavities, as in blow holes, are hidden from sight. The great necessity of uniformity of thes field density is onily in one direction. I would like to point that out on the board. (Illustrates) If You have a disk which rota- ting in its own plane, and which is represented by that circle (re- ferring to a sketch) then it does not rnatter how the flux density varies along any radius, it only matters how the flux density varies along any circle. If the flux density varies along anly circle, then there will be a tendency for the eddy currents to produce thlemuselves. But the flux densities nay varay from the center to the periphery in the most arbitrary nmanner, provided that whatever variation takes place at this radius is symiminetric- ally reproduced all arouind. The formn of field-magnet wlich is presented in this paper and wihich has struck Mr. Hlering so forcibly, as being unnecessary, has probably, I presume, been introdnced with the idea of con- struieting a gigantic maelline, because the larger the mnachine is, the less is the fraction of the total reluctance forrmied by the air- gap, and the greater is the value of the iron in the immediate vicinity of the air-gap. I presume therefore that all these designs are intended to be used on1 a very large scale in practice. MR. STEINMETZ:-In this problem of ulipolar malachines we have to be very careful to guard against a mistake whieh is madec quite frequently, especially in tr-yinig to reverse the action of a, machine. In an electrical condnietor an electromnotive force is in- duced, if it moves through or relatively to a magnetic field, that is,. eithier if the niagnetic field varies in intensity, or if the magnetic 1894.] DISCUSSlON. 42 field is constant and uniform, and the conductor m(ves tlirough' the field. Now take this uniipolar macihine in Fig. 3. Tllere is a, constant magnetic field, that is, a field which has at any point in space a constant intensity anid constant direction, no mnatter whether you revolve or do not revolve the magnet; because if you abandon this idea of lines of force, which is niotlhing else but a physical hypothesis, anid go back to the original ineaning ,of the magnetic field, as the inltensity and the direction of the 1 agnetic displacement in the ether, you can find that in the m-a- eline, Fig. 3, eveni if you revolve the magnet, the field is con-

FiG. 9. stant. Thlus you can very easily get a maclhine which can never do anything. Take the machine that Mr. Stanley showed us Fig 9. Whetlher the mnagntet and the m-iagnetizing coil stand still or whether they revolve, the intenisitv and direction of the mag- netie field at any point is conistant. Hence, there is no E. I. F. induced in the stationiary armnature conductor. Still you get a cuLrrent from- the maehine. But wlhere is the seat of the electro- motive force ? It is where the lines of force are cUt. That is, inside of the revolving magnet, in the line A B. The very large diamuetev of armatures necessary in a unipolar machbine is no serious objection ; because in direct-coninected we have runniing in this countr-y arinatures of 12 428 CROCKER AND PARIL Y ON UNIPOLAR.D YNAMOS. [May 16, feet and 16 feet in diameter, though not with 200 revolutions by any means. But where I see a more serious objection is, that I fear the efficiency of the miachine will be very seriously reduced by the brush friction, because you need an enormous brush to take off the current, and even if you use very little pressuire of the brush, still you must consider that you have a speed of five -to six miles per minute or over 300 miles per hour. These are tremendous speeds, about three times as high as the highest speed reached in revolving dynamo machinery, and about eight times as high as the highest brush speed on collectors or com- mutators. As to taking off the current by imiercury contacts I have never tried it myself, but I remnember some remarks of Professor Forbes about it. le said he lhad tried it and had been -very unsuccessful. fHis statement was something like this: That lie could not get any currenit, because before he could get any cuLrrent the mercury had evaporated by friction. I tried myself some years ago to build a unipolar machine to solve this problem of multiplying the electromnotive force by

N N

FIG. 10. FIG. 1 l. oiling the conductors, and I succeeded witlh it. It was very successful indeed, only when I was through it and saw wlhat kind of machine I got, I was thoroughly disgusted and gave it up. I think 1 can show you the chain of reasoning. rt is quite iinter- esting in showing what a unipolar mnachinie s eculation can lead to. I started with Faraday's disk, Fig. 10. The magnet is N S. Then electromotive force is ind ced, say in the direction of the arrow. To multiply the E. M. F. I took a second disk. Now, connecting the outsides of the disks with eachi other, in Fig. 11, I could return the E. M. F. of one disk by the other, and thereby bring the two brushes to the center, as shown in Fig. 11. By doing this! I have to revolve the part of tlhe magnetic circuit between the disks, that is, make a kind of an armature, consist- ing of an iron ring between the two Faraday disks. Evidently these disks have to be slotted to get rid of eddies. But if you slot them you are enabled to coil the conductor, as shown in Fig. 12, by offsetting the slots, so that the eircuit leads from one segment to the next, and thus you have so]ved the prob- 1894.1 DISCUSSION. 429, lem of coiling the conductor in the unipolar machine, but if you look at what you have, in Fig. 12, you see it is nothing but the regular bipolar dynamo. Sinice that time I gave up trying to improve ullipolar machines. THE PRESIDENT:-I will ask Prof. Crocker to close the dis- cussion. PROF. CROCKlER:-In regard to what Mr. Stanley has said, it seems to me that, as Mr. Kennelly stated, it is simply the ques- tio-n of the alternating versus direct current which is not the subject uinder discussion. But taking the case as Mr. Stanley stated, we slhould have there all the coimplications of armature windings. YouL would entirely lose that simple and solid arma- ture construction. You would require insulation, etc.

S N S N

4 N 4E-~S N~ S

FIG. 12. Youi complicate greatly the mechanical construction. Further- more, the alternator certainly wouild not do for electro-metal- urgical purposes. It wouild not do to-day, for instance, for a central station, where they are using direct current and want another dynamo to give a larger output. This paper relates simply to unipolar direct current inachines, and I was not coin- pariing them with alternators. I was comparing them to the ordinary type of dlrect currenit machines. In regard to what Ir. Steinmetz hias said, those ideas are ex- actly the things I wanted to avoid. My experience has showin me that when we depart from this simple disk or cylinder con- struction, we get into all these complications and fiually arrive at a result which is very inferior to the ordinary Gramme o01 Siemens machine.