Multiphase Motors And The Transmission Of Power

Saturday, September 9, 1893
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193 September 9, 1893 THE ELECTRICAL WORLD, vm. xxu. s.».u. Illultlpllaee Motors and the Translnlsslon of Power QUESTION so tmportanl as the long rllstnnce trans- mlsslon or power had been apparently overlooked ln the prvpnrntlous for the 'f ‘ rr-cent lntomntlonnl Elec- trlcnl Co|\::ress, ns no !» _r ' V"', papers on this subject ap- f~ peered ln the programme. and thls fact excited _ more nr less comment. When lt was understood that the matter would come up for conslderntlon In tho scctlon of Pure Practice, over which Prof, E, J. llouston pre- sldcd. lt was evident that the discussion would be fol» lowed with great lnterest, nnd that the |\tte\\d:\nce ln Sectlon B during tlmt time would he very large. The Idea. lt ls stnted. orltzlnnted wlth Dr. Louls Duncan, of Johns Hopkins Unlvelslty, who has always taken an nctlvo Interest in lhls question, and ls rcnnrdetl as one ‘of the lendlxx exponents of the trnnsmlsslon of power by nltwnntlng currents ln tllls country. -On Thursday forenoon the clmlmmll of Sectlon B ac- cordlnnly culled upon Dr. slon, and as many pnlnts of great vnlue nnd ln- terest were brcu,=:ht out ' lt ls clven below ln more detnll than the brief syn- opsls which appeared lu the report of \l\e pro- E WL U \ ‘ =\l, _ 57% =,§§»3;‘.ir; ,A __r he ~ "TW * D\\m'|\\\ to open the discus- §,§§,._:_ _I . J.-\ V; .` U . _ ` W. lteedlngs of the congress -, ' \,._.,-» ; Electrlcol World, " ~*;.. . Dr. Duncan ilrst ex~ " ‘ A plalned why the discus- cusslon had arlsen; It ~, was found that then- _ -» were D0 3 ers on uh' [- P D _ fzc,w.,,qr»~, .r 5 4 J -\ ,_ _:xi ln the last lssue of The » §‘;< -% _ . ,» ‘H sv l Y llst of those to he reml before the congress whlch touched upon the subject, andns agreat runny men, who had n large amount of pruutlcul experience, and who probably knew n great. deal more about nxultl- phnse systems than any one ln thle country were present at the congress. lt was desirable, lf poealble, to get hold ol their knowledge and experience, ` Dr. Duncan thou brletly descrlbud the theory of the multlphase motor, and showed that the dlstrlbutlon of E. M. F. was such as to give |\ higher etllclency than ls pomlble ln dlrcet current mnchlnes, Tnklng the case ot two independent nlternntlng currents ,with n dllfer- ence of phase ol' 00 degrees, one ls nt zero when the other ls at maximum, nnd If these are led around the mag- nets of n four-pole dynamo lt ls erldent that a xnaxl- mum lnduccd ln cnch wlre when lt ls placed ln the best posslble posltlon ln the deld of natural strength. There ls, then, n dlstrlbutlon of electrical force ln Lhe conductors about the annature, such that the hlgh- est elecirlcnl force ls ohtulned exactly where lt ls de- slred. The dlstrlhutlon ls such that the current l.s ob- talned exactly where we wnnt tt, and for that reason the cnndltlons are better than ln u contlnuous current maehlne. lt has been found ln the Bret plnce ln work- lng urp the problem tlmt the eleotrloal efllclcncy of such n motor le a very elnrple cxpresslun. equal to thc angu- lar velocity of the nrnmturc dlvlded by the angular veloclty of the tleld, Such n motor would start up just llke n continuous current motor, and would have an etilclency greater than 11 continuous current motor, provided there were no lamrlng of the current The stnrtlng of the motors ls the most scrlous questlnn which has to be dealt wllb; ll we have a motor where the drop of potcnllzxl ls 10 per cent. or something llke that, nnd lf the xnnehlne when starting has a cun'ent of mzmy lhnes the normal current. the whole dlstrl- butlon ls dls:\rr:\n;zcd. lt ls just as lf a contlnuous cur~ rent motor were started with the brushes shifted almost 90 degrees from the neutral pnlnt of the cnmmutntor: Tlxequestlon\s.HowIs|t possible to get rld of this trouble? The llrst lhlnn to do would be to dL>C\'Pase the self-lm duction; this has been done by the Stanley company by plnclng short-clreultcd colls ne:\r`the nrmnture dolls; but this ls done nt the expense of a :ood deal of space whlch could olhcnvlse be fllled wlth Iron. and lt ls questlonnhle wlxetlxcr It would not be just as well to utlllze Lhat spruce by puttlng more Iron lnto lt. Another way ls Io increase the nmmture rcslstnnce of the motor. There has been n xrrcnt deal of trouble wltlx multlphnse motors, and one of the reasons for lt ls this: Suppose that n wave Ls mppllcd that ls not a slne curve, but ls a positive and fundamental wave, a number of upper hannonlcs; suppose \ve have the E. M. F. together with the same hnmxonlcs, then the etllclency ot the motor would he greatly reduced. It the nxotor were revolv- lng at 000 revolutions nnd the fleld nt 1,000 the electrl- cal cftlrlency would be 90 per cent., but ll more waves are t.\\ere, say. the thlrd nr tllLh harmonics, then the etilelcncy ls greatly

198 an Important polnt, and the lower linxlt. has never been thoroughly investigated, The thst effect of reducing the frequency ls to dhnlnlsh the poles in the dynamo, and lf we go down far enough lt ls necessary to slo\v up the machine until we have a dynamo running at n small speed, and its outlput is nut what ls desired. Again, it two sides are not perfectly fretjuency, n tremor occurs creat vibration of the ma- lmth dynanlos and motors. frequency necessitates ln- ln a multlphase system the balanced with a very low which may produce' a very ehlne, and this applies to Further, the decrease of creasing the slze of the transformcis lu a nxtlo some thing like tire to one. It' it is desired to have motors running at 1,000 re\~<»lntlons per inlnnte the limlt of the number of pl-rlods ls smut-\\'l\ere about 20. ltr. Louis Bell then eolisldered some further poluts lu addition to the 11-niarlis he had already made. He awured Dr. Duncan that the effects nt’ the wave shape on thc operation ut' single or polyphase motors was au experlutental fact, but that ln practice the etfcct on the erllrleucy was not likely to he great. with well desltcued nnxehlnes. In regard to the regulation of intercon- nected clrrult pulypllase systcnls, hc had found it quite easy to satisfactorily regtulate tl|e trlphase inter-dependent rircult system, lt was lunch the same question that had arisen when the tlirec-wire direct current sysletn had been introduced, and the same objections were made against it. As to rotary flHIlSf0l‘lll(‘l’S, he was not as hopeful as Prof. Forbes that any more simple form of conunutator would come into general use in the near future. The rotary transfonncr has an effl- cieney from 90 to U5 or Dil per cent., and ls u |hor» oughly reliable and eolnparatlvely slmple lplcce of up- paratus. He fayored -the use of step-up transfnrnxers rather than dynamos of very high potential on the ;rnuud that there was no experimental knowledge in the latter ease; for extremely large machines he thought, luvwevr-r, that they might be built for very high volt- ages. In ctu\side|'inL: the line two important factors were the lnduetanee and the frequency. As regards the lnductanee, tl\c trlphase or polyphase interconnected systems, forthe same energy transmitted at the same voltages, give a lower Induction on the llne. \Vlth the t.ri‘[)hase interemlnectcd system the lnduetanee ls n llttle less than .G of what lt is ou t.he slnglephase or Independ- ent clrcuit nmtdtiplxase system. As to frequency, tlze upper practleal limit would lprobably be somewhat less than the frequency new ordinarily used, that is, 125 to 130 cycles. The lower lhnlt would be determined in the or- dinary dlstrlhution plant by the necessltles of incan- descent lighting; below 30 periods, or, ln fact, below 33 periods, Incandescent lamps do not work well. Arc lamps with the best soft cored earbons work fairly well at 45 to 50 cycles. The practical lhnlt, then, is proh- ably from 30 to 35 cycles, The keeping up of an over- head line is a most.serious problem. The subway ls, on the other hand, ln most eases absolutely prohlbltlve from a colnntereial point of vlew. He thought that in most of the overhead work bare overhead wires must he employed, and, lf they were dangerous, that people must learn to keep away from them. The lhnlt ol' distance to which po\ver could be transmitted practically depends almost entirely upon the condlilons. Given the price of rual hlgh enough power can be transmitted to ahnost any dtstance. lt ls absolutely necessary, however, not to economlze in the matter of insulation. Dr. Louis Duncan, ln closing the discussion, stated that the tendency of most of the foreign engineers and those of the American speakers wha. had had no prac- tical experience was in the direction of the single-phase system; but that those ainung the Americans who had had considerable experience, and had seen the advan- tages of the mnltiphase system, were ln favor of it; cer- tainly the inultlphase system at present was thc only practical systmn for general distrilmtlon. The question of frequency was one of vast importance. If lt is intended to do arc or incandescent llghtlng, then 50 periods per second must be used; if lower perlods are used, the lag of the current' is greatly Increased. ._l__,... THE ELECTRICAL VVORLD

ssnmssn mms. THE ELECTRICAL VVORLD. \97 called upon to continue the dlscusslon, He stated that. as oonsultlng fenglneer, he represented n. vnrlety of oompnnles ln Cnllfornln who dealred to trnnsmlt power electrlonlly to the nggregnte amount of some 40,000 h. _p.- over distances ranging from 10 to 44 mlles. ‘Iot- wlthsnnndlngihe Inc! that Mr, Scott had referred tn the successful tmnsmlsslon of power tor llglxtlng pur- poses nt San Antonlo over 28 mlles, the tr.msmlsslon of power electrically over long dlstnnces ln Cullfornlu was n tnllure. Over xx. year ngo the tlx-st contract for 11 dellnlte trnnsmlsslon of power was 'closed, and today the superintendent of the mine In which the Insmlln- tlon wus made ln enstern Cnllfomln ls looking nt. hls bumedout tleld colls utter the generator has been ln operation for less than Lblrteen dnys, The whole sys# tcm just described ls logical, and very beautltul on paper, but lt dons not estnbllslx the wmmerclal Eno- oess ot long dlstnnce tmnsmlsslon of power. The mn~ chlnery must be so built that it wlll not run for hours or days, but for weeks und months. Insmllntlons of this kind ure nlmost lnvnrlnbly ln plnces which nre dls- mnt from rnllwnys, and where repairs are lmposslble, and the mnchlnery must be of such n kind thnt. lt wlll be practically lmposslble tor nccldents to happen. Mr. Hnsson stated that ha belleved ln the trnnsmlsslon ot power, und hnd been nn apostle of it for some time ln Cal- lfornln, but that he was 'obliged to dellnltely state that ns fur ns Cnllfomln was concerned the results had been failures, at lens! from the stundpolnt of the investor. Some months ago he was culled upon'to decide upon cel-tsln plans for the transmlxlon of 3,000 h. p. for 20 miles; mcse elaborate and beautiful descrlprlons of methods nrrlvtng nt the results were placed before him, but not a slngle working drawing of any klnd. Dr. Louls Bell then took np the subject and tlrst ex- pressed sotlstnctlon that ll previous speaker had re- ferred to the-two~phase system as n. polyplxase system, because he thought that all which applies to the two- phnse system applies ln vnrylng degrees to all other numbers of phases. He stated that there are ln Eeneml three methods which nre usually proposed for power tmnsmlsslon-t.hnt of the dll-ect current, whlch lfx-of. Crocker had been chnxxiplonlng ln his paper before the Congress; that of single-phnse currents and thu! of poly- phase currents. In regard to the tlrst, lt could only be said that ln thls country, at least, lt had not been alto- gether o success; the great dltlluulty seems to be that when the current lncrenses to any conslderubla extent dlmcultles are nt once to be met with. As for nlternnt- ln: currents, the reason why they nre preferable in thul. they can be utlllzed ln so xnnny dllferent ways. So long us the 'dlrect current had n, monopoly of all motor service there was no choloe, but at the present day that ls changed; ,the stnglophuae system ls now applicable to motor sérvlce, though not as well ns the polyphnse system, nor is lt an well adapted to the oper- ation of the rotary transformer, which is destined to 'play an important port ln the future tmnsmisslon by polypbnse currents. The single-phase motor ln lt.s-pres- ent state of development cun be considered ns nothlng else than a rather poor polyphnse motor; lt starts on the polyplmse principle, but with the objection that the rotary ileld eatubllshed ls not u clrculnr rotary tlcld but nn elllptlcul ang, amd, consequently, gives less elllclency in the motor. Corning dually to the polyphnse systems, they may be dlvldedlnto two classes, those havlng two wlres per phase or' Independent clr- cults, nnd tliose lxnvlng non-Independent clr- cults. A two-phase system with three wlres ls some- what slmpler than the two-phuse Independent clrcult system; the threephnse system, opemteti on separate circuits, requlres slxfwlres, but.wlth comblned clrculls requlres only three wires-ns few' as are requlred by any polyphnse system. Funhermonz, ln all the poly- phuse non-lndependent clrcults polyphnse systems savé copper, ns over the direct curnentv the slmlple alternat- ing and the polyphnss system with separate clrcults, and this snvlng vnrles ln degree from 12 or 13 per cent. Lu the twtrphuse to~25 mer cent. ln the mreephnse. Another important feature of the Llxreephnse depend- ent clrcult system ls that there ls less self-lnductlon and capacity on tlie llne than ln single or two-phase systems. For the Installation of lights the difference be- tween n dapendentclrcult system with three wlres and an independent clrcnlt system with at least four wires ls very llttle_becnuse lt ls necessary to spill: the cur cults in any case, but ln most cases three wlres aro more convenient than four. The Krent objectlon whlch has been mlsed against the polyfphnse system with dependent clrcults ls that lt does not regulate well, but this crltlclsm Ls lll founded, ns we know from experi- ments. In the llrst place lt ls quite posslble to arrange such n system so that the dltfercnoe lxetwccn the dlder- ent branches of the clrcult wlll be almost uuuotloenble. The means of doing this are slmple, und therefore lt ls well to state as the result of positive experiments that without any compllcabed means of compensatlon wlu\t~ over lt ls possible to obtnln closely concordnnt volt- ages in all branches of polyphase system. In regard In polyphnse motors, there la no very wide rlllferenos in prlncllple or ln opemtlun between two, three. (our, or m phase rnolors or in their action on dependent nr independent circuits. When such inotors Au'e'pr0pe‘ly»du@¢d then ls no doubt that they no decldedlymupcrlor totnny dlreet. current muchlne, und that ls one of the strvng reasons for using the. polyplmse systems. Dr. Bell atnted that he had been experlmentlng with motors of this type for n long tlme, and that he had never yet succeeded, even under the most adverse con- dltlons, ln burnlng out n single mllchlne ln nny wny whntever, u record which could hnnily have been made with :my kind ot n dlrect current motor. If overloaded they wlll stop, and not bum out for n pretty long pe- riod; they start qulte well, ns wcll ns nny ordlnnry dl- rect current motor; and ln regard to the cun'ent re- qulred, they start just about ns n shunt motor does. lt ls possible to run n polyphnse mo_tor at n wldely vnrylng range of speed (soy, from 1-l0\h speed up to full speed), keeping a, prnctlrnlly constant torque. Dr, Bell, in closing, defended the polyphnse rnncblne ngnlnst the nocusatlon of 11 lnrgedngglng current. Prof. S. P. Thompson, llrst speaking for M. Thury, briefly descrlbed the lrnnsmlsslon plant nt Genoa, where 21 dlrect curernt of 1,200 volts was employed and no dllllcultles were experlenced. Another lntcrestlng con- tlnuous c\n1‘ent plant ls that operating between Blber- lst and Rondchntel, n. dlstnnce of about 20 mlles. Forty nmperes of cnnent st 3,500 volts are there transmitted, At Genoa elgbt dynnmos, all jolnedln serles, each run- nlng nt nominally about 1,000 volts, supply generators. These are all samples ot prnctlcsl lnstnllatlons fur which M. Thury makes himself responsible, In speaking for himself, Prof. Thompson ilrst touched upon 'the continuous current problem, stating Qlxst many contlnubus current machines of high voltage had been successfully operated, and cltlng cases ns illustrations. Coming to the question of nlteruatlug current work, he cnled pnrtlcullur attention to (he lnstnllnlloll nt Rome. where the source at power was xx waterfall some four- teen-mlles distant. He land studled the polyplmse lrnns- mlsslon nt Frankfort, where there were elglxt or ten dllferent systems on exhlbltlon, from _nll the great llrms ln the world; he dld not believe that there was any best system, but that each one was best for its own pur- poses. Ho dld not cam to prophesy, but would glva his personal oplnlon ns to what would be the Keneml sys- tem In use ten yenrs from date. He dld not llke the complication ot Llxree-phase transformers, and three- phase swltchbosrds and other nppn.mtus, although he dld llke threeqahnse and two-phase alternating current dynnmos und motors. The- system recently described by a previous speaker was very beautlful, but he dld not llke lts complications. In the future he thought that all the nppllcatlons of polyphnne abandoned, and that engineers would alternating current motors; ln other simple nlternntlng current wlll ,be the ten years hence wlll be found to be long distance trunsmlsslon. work would be retum to simple words, that the one thlng whlch etlectlve for all Mx. 0. P. Stelnmetz remarked that out of justice he would be obllged io tnke the part of the continuous current; the largest amount of power dutxlbutpdiln the Unimed States to-day ls distributed by continuous cur- rents; this shows that the continuous current- ls not dend yet, and the many' referemes to rotary trn.ns~ tormers showed that even the advocates of the polyphnso systems stlll conslder that they cannot get nlong \v1th- out the old continuous current. Wlth regard to the polyphnse und. slnglc-phnse systems, lt mnkm little dll'- terencu how many phases there nre, because nny_ sys- tem ot,polyphnse currents can bc transformed lnto nny other system ot polyphnse cnnents hy uslng two trnns~ formers only; furthermore L\1e'motoxp of the different polyplmse systems_ are essentially the cnme. Mr. Stein- metz Thought that the system of the' future would be the slnglephuse system., There would be no longer any trouble wlth self-lnductlon or eupaclty, for proper nppll- nnces would ellmlnnte their eliects entirely. Prof. George Forbes suited that he was lnlormedjhnt there had been nn organized conspiracy among the Amerlcun englneers, led by no lea a man than Dr. Louls Duncan, to dmw out the foreign el\Bfl1eQ1'¥ on the subject ot tmnsmlsslon of powerand multlphnse motors; he dld not think that this referred to him. as he has ceased to look upon himself sn n foreign engineer. Prof. Forbes then proceeded tovoullhla the methods which had been ndopted tor the _great trnnsxnlsdon blxlnt dt,-Nlngnm Falls.---The snbicct had been lnvestl- gated most. Lhoroughly by himself and others connected with the enterprise, l.n order tlxnt the best system might be selected. The llrst step taken was the dedslon that there should be central stations esmbllshed nt Nlngnm Falls to trnnsmlt power to places at greater or less dLs~ tnnces nwny, as opposed to the pmposltlon of locating all the u1llls'd.lrectly nt the Falls. lt was dcclded tlmt the whole of the trsnsmluslon should be done by menus of electrlclty, In 1890 n number of plans were lnvlted from dldferent engineers and engineering llruxs ns tn the best means of utlllzlng the large power ot the Falls, xmd.tlxese plans were submltted to lm engineering con- gress, which met ln London at the beglnnlng of 1891. Prof, Forbes ln his report nt that time. stnted that he regarded the bei! means to be that of transmission by means df alternntlng currents of eleetrlclty; that Lhe two-phase system should bs employed wlth two sepuate clrmlh of high voltage; that transformers 'should be used for reduclng the pressure doum to u. safe llmlt; that ln the xullls und works syuclxrnulzlng nltcrnullng current motors should be uscd ln some cases, and ln others two-phase motors should he employed, nnd that ln cnscs wherc direct current was necessary nltcmnt- lug motors should be used to drive continuous cnr» lent machines. During the three yems since tlmt time hurrlly 1| clulnge has been umde ln the system proposed. After the congress referred to haul closed, bids were nsltvd fnnu ull the greatest flrms ln the world. The pzrvntest dltllculty wus cxuerlcncud ln ucurly every cnss by those who prorposnd to use cnntlnuous currents, mud lu on-ry case the cost wns largt-ly lu excess of that of the nlternntiug system. Another thing which had been dcvldetl upon was to use the same system for both short nnd long trnnsxulsslon. The ntl\'nnt:\ges, lxmvever, ln the use of high potentinls for short lllstnnccs nre the some ns for long, and ll wus found that lhc nnxouut of copper wlllch would be neccessary to trunsmlt 50,000 lx. p. nt n poteixllnl of 1,000 volts over even n short dlstnnce would almost completely fill 11 subway large enough for a man to wnlk through, and the cost: would, of course, be prclxlhltlve. Prof. Forbes hardly ngreed with Prof. Thompson tlxnt the multlphnse xnolon \vould disappear from general use, und tlmt slnglephnse motors wltlx n m\1lllU>h:\se menns of stnrtlug would be more unl\'ers:\1ly adopted; this he regarded ns only one of the pnssibllltlesbf the mppllcahle to nlmnst every cluss of work. For continuous work, such us wlll be found ln mills, the synchronous motor would he employed ln the Nln§1\-fn plant, but In all ordinary workshop practice where lt > ls necessary to stop, start and reverse continually, .the multlplmse motor would be employed. One reason' why the two- phnse independent clrcnlt trnusmlsslon was chosen for the Nlngurn plant was tlmt lt was llkcly to tnke care of all developments whlrh would be mnde for the next few yenrs ln nltemntlng current nppnrntus; even It single-phusé machines nlone should be used the single- plmse currents can be developed more cheaply by 11 two-phase mnclxlne than by the onllnnry slnglc-plmse generator. This was npprcclnted ns eurly ns 1ST9, when Mr. Grzunme bullt lnls llrst nltcmntlug current dynamo with two Iphuses, hnvlnr.: eight polcs, n revolvlng Belt) and u. llxed nrmntnre. The water of the Nlngam lllver, ns ls well known, ls tnken oft one mile above the Fnlls by a large cnnul, \vhlcl\ hns been bullt; lt ls then taken by tunnels into the wheel plts nnd sent down on pcnstocks to the depth of 140 feet to the turblnes below. These turblnes have been designed by o llrm of Geneva. of world-wide reputntlon, und have been cow structed by the I. P. Morris, of Pl.\ll:\delpl.\ln, and will be dellvered shortly. The wnter, nfterpnss- Ing through the turblnes, ls czlrrled down through the tunnels, wlxlch have been built to the rlvcr below the falls. On the top of the turblnes ls n vcrtlcnl shaft com- lng to the surface of the gmnnd, nnd rotating at 250 revolutions per nxlnute, which enables lhe large dyno- mos to be directly connected to the shaft wltllouttgenb lng. Prnt. Forbes then took up the questlon of whether step-up trnnsfunnem should be used or dynnmos gen~ erntlng n current of very hlglx potentlnl. Ile esthnntcd that, with 11 loss of 3 per cent. in the transformers lu the 5,000-h. p. unlts, $3,000 por yenr would be saved per unit With stntlonnry nrmntnres It was as easy to build n dynamo for very high potentlnls. ns It was n trzuxsformer Itself. Overlwzld llnes, sul»\v:\ys or colt. dults were the three menus by which the current could he trnusn\lttc¢l, mul lt was u very surlous ques- tlon ns to which should be used. The subway wns ob- viously the most satlsfnctory, but the pole llne was the cheapest. It had been decided that the work would be done elther by bnre overhead conductors or by bare wires currled ln n subway. The tnxnsnxlssion ot' [power to consldernble distances from Nlaxgnm Falls \vould confront tho company ln the immodlnte future, and lt was possible tlxut electrlnlrv \vould be used In propel the canal bouts on the Erie Cnnnl, ln which case n llne of sevcrnl hunrlrcd uxllos ln lcnglh \vould be rc- qulred; ut nny rnte the trnnsmlsslon of powcr to Buf- falo wns u posslblllty of the nenr future; thc expense of ll subway over such dlstances would be of course, very large. Experiments would be cnrrled out with the pole system to determlne,lts ren] merlts. In the meantlnxe u suhwny hnd been started which would curry conduc- tors from tho power stutlon ut least ns flu- ns the Pitts- burgh llcllucflon \V0rks, about hnlf 3, lulle dlstnnt. Prof. H. A. Rowluud dld not ngrec with Prof. Forbes that dynnmos of very high potential could be con- much rlllllculty. Ile was inclined to step»up iluxxsfoxuncls. The snmll loss much consequence \vl|crc tho source wulor po\vl>r nt' such cnfuwlly. The structed wlthout mvor the use of \vould not be of ol 0\\0l`|(y \Vll§ l\. dltlimlltles that \vould he vxnzmxnlt-rur- ence of potentlnl botwocn any zuljzlcent wlres. Then ngnln, ns the potential lncrcnsos, the nmouuf of wpcr which can be got into the groove ln the nrnmture be- comes less null less, The matter of freqnnncy wn.s also