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Hero the ditJeronco of phase between an_y two ciiirent ; is " , There are several methods of ohtaiiiinLT, frnni a single allernat ing cui'rent, two alternMtlMg cui'renfs diHering in jiliiise, proihic ing ii rotating mngnclic field The single curienl is s))lit ujiinto two liran(dies, into om(( of which is in-^'M'tod ii small resistance and great inductance, into the otiiei' a largo resistance ami small in- ductance. Tlio phase dirtoience in this case nuiy a)i))roach 0(»\ hut will always tie snudlor. Let us assume it to ho 15", tiir in- stance, then the resulting curve will he the ellipse of Fig. 1, sii])- posing. of course, that tho ampiMo windings in hntii coils have been made equal. This method gives great fluctuation in inten- sify of magnetism of tlu^ rotating tiidd, a thing to be avoided. Another method, u|ion which tlie writer has spent consideiable time in endeavouring to devolope, is to split tho single current into two and introdiico into one liranoh a condenser. This has the efl'cci of giving an advance dilVorence of phase of 00°. The dilticully with this method, which has not as yet been overcome, lie^ in the fact that a condenser of sutlicieiit capacily and insula- tion has not been brouirlit forward. Mi-. Teshi, in his latei- motors, ma lies Use of til e first of these methods, but the conditions for coMstaiiry of velocity and inlensity of the rotating liehl ai'c vciy imperfectly fulfilled. In his lalist design of rotai'y current motor a four pole machine is chosen. Miglit radial olcctromagnets are placed at ei|Hal distances on the inside of an iron -ing, !iii(«. 1, (J and 7. Inter- oslin;,' roMiilU wuio oblaiiiud for llio tlirou coil motor showing lliut, whiitovor ijiu jiusition of tho lotatiiif? miignotic field, the miignulic iixis was goneiully ciiivud, and iho lines of force con- tructod ut one polo and r-pieud out at the other, lliat U, tho in- tensities of tho two i)(,k.rt were nevei> the sumo. Tlio four-coil motor gave a con.Mderably bettor result. Tlio magnetic axis was always straight, and both polos had equal intensities. The six- coil showed a still more equal resulting Held. Tho intensities of the rotary liel.l was obliiined by liaiiging a roil in tho centre of the ring, in such a way that its nuigiietic a.xis was perpendicular to the measured direciiun of the resulting magnetic axis of tho ring. Tho coil was then excited by a constant continuous cur- lent, and was kei)t in its position by a spring. Tho tongue of the spring served as n measure of the intensity. Tho horiozontal compound ol tho magnetism of tho oarih was neglected because tho ring surrounding the measuring coil eireetually served as a magnetic screen. In this way they lound that the mean intensi- ties in tho throe, four, and sixeoil motors were as 117 ; 127 ; 18(J. Tho rtuctuations ol inlensiiy were not appreciable in the six-coil motor. In tho tburcoil motor those fluctuations did not quite reach 13 i)er ceni. of their minimum value. In the Ihroe-coil motor tho moasuremonts of tho inlonsities were not very valuable because tho resulting axis was generally bent. The fluctuations of tho angular velocity of the lesnlling axis wore much longer than the fluctuations of tho intensity in all three motors. In the thrcecoil motor it was found that the two p(des always moved without (lifloronl velocities. Since the velocity of the polo,s them- selves chiefly influence the armature, it follows that this type is certainly not very advantageous. In the tour-coil motor the variiitions of velocity are veiy much smaller, and are still smaller in tho si.x-coil typo. These experiments are of great value in that they throw a good light on tho fluctuations of tho rotary current lield. Tho result of a rotating magnetic Held in the interior of a ring act almost as if a constant magnetic field wore mechanically re- volved in it. On this principle extremely simple motors, with- out either sliding contacts or bru.shcs, can be built, and the use- fulnoss of largo mot(jrs, having these adv: ;ages, has been one of the stronge.st reasons for experimental work in this branch. It has been stated that the conditions for constancy of velocity and intensity of tho rotating field are very imperfectly fulfilled in the Tesla motor. In fact tho excitation of the motor varies as much as 40 per cent of its minimum value, so that the magnetic field is not merely rotating, but also pulsating to a very considerable extent. Tho action of the motor is accordingly far from being simple. The typo of alternating cuirent motor designed by lilihu Thomson affords v.-' an example where the two properties ol the rotating lield may bo considered separately. This motor consists of a magnetic field duo to an alternating current, and an armatureof which the windings form clo.sed circuits. Like all synchronous alternating current motors, it must bo brought up carefully lo the proper number of revolutions jior minute before tho altornat ii.g current lakes up tho work. If tho work be thrown on too soon, or if the motor be overloaded, tho effect is to stop the machine. As ihe pulsations of the Held only serve to turn tho armature when it is going at a particular speed, and as the cuiTonts induced in the cl,)soa circuit armature do not corres- pond properly in lime and direction to those of tho field it is very dirtb^ull to start such an armature in its alternating'fiold When a two-phase motor, with closed circuit armature such as Tosla's is not running sy nchronou-ly the torque is only equal to Ihe dirtercnco between the efiect of its rotating field and the checking duo to the jnilsations of the magnetism. The motor therefore, does not run well under load. At full speed it has a 6 toiuicm-y to synoliioni«iti uj) to a corlnin lonil, lioyond wliipli Un N|i(<('(l and li>r(|uu lupidly iliniiiiiNli. Tliu UMd'iiliioM of hiicIi u inotiir Ih limiU'il, und it cun only lie rcgni'dod ii« an imiirovomont on ttio poiloctly NynchrniioiiH niciloi' nl' Thiim>oii, liy loason of its power to Ntait iindci' a licavy |(iad. To |iiddiu'i> a i,'()oil motor wo niu-t diniitiJHli tlif aini'lilinlo of tiio jiiilsutionH of till' alloi'iialin;; inagnotiNm of llio Held. An ideal field would bo without nui^netiu pnlsulionM. TIiIh idoal in np- proaclied l)y iiieroaNinn llm niiniliei- of cuii'tMits, of wliich the |)li!ises are M'adu In follow one aiiolhei'. Intlioca>eof tiirou eurreniK, Kijj. H, it will be neon that the alfjehraie HUm is Huffl- eienily eonHtanI, tlio tops of tho curves represenlinj; approxim- ntely (ho values of the tliietuiitidiis. A simple caleulalion shows that Iho imlsadon of lliis lield is only ITi jh'I' rent instead of 40 percent as in Tesla's motor. Since the nmnlier of allornations is relatively small, the salnrntinn of the iron can lie carried up to a hi^^h deforce without impaiiin;;- the eOieienc}-, so that foi' a variation of 15 percent in tlie ampere turns, tho total (jiianlity of maf^'ictism remains practically conslanl. Willi four alteinal- inj^ currents dillorin}^ in phase the vuluo of the pidsations would ho still further reduced. Hut hero wo are mot by the fact that the increased numher of ('(mdnctinf^ wires constitutes an oh-'taele to the practical a|i]ilication of iheir transmission. I'.ven in the eu-e of tlu'ee currents wo would re(|uire sit least four wires, which would still he too eo^tly. Viui Dolivo-DohrowoUky has work«Kl out an arran;^oment hy which three currents can lie eonveyod by three conductors lie supplies three currents ditVeriuf^ liy 120" instead of (iO° in )iha>e, and reverses the connections of the bob- bin whieli supjdies one current. To each of the circuits in tho motor corres]iond at least two oppositely wound coils produeinj; o]iposite poles, so thill he is able to add I he currents obtained by re- versin;^ their si;.;ns. This results in a tij^cnre i|uilc aiialoijous to Fiif 8, which roiresents tho currents ditVei iiif,' in phase bj- VM" instead of (50°. Ho linw two methods of Joininj; tho sections on his ring, one similar to Kiff. 4. wlii(di he calls a i-i->H'il connection, and the other similai' to Fig. !•, which ho tornis an njhn connection. \ f '"'^juirr-. These connections render )ioBsililo the conveyance of the three currents through tliieo conductors, in that thecurrent flowing in one conductor finds its way liacif by the other two. A similar system can also be employed in the ease of more than three cuv" rents. Von Dolivo-Kobi'owolsky states that in tho course of care- ful ex|iorimciits cari'iod out by him with the Alli/emeiiw Eleclitri- I'itiits (legcllschaft, and while working out the del ails of his system, there ajipearcd many cogenf reasons for abondoning lliis Kim|)le arr.'ingenieiil. Anlollg.^t lhcs<' were the small output and con- sequent low efficiency, the heavy cost of dynamos und motors, and thedifliculty of ie in< roaneH ; iifler tlio mnximum luiM lioi'ii ri'iiclii'd hotli ilcciouK' in A, but inciTiii-o in 11, ittiil ho on, Hiiclly, till) iniiKniti«ni tiuvi'lH fioni A ovor H to C, thus couiplc,' tliu lyi'il'. Hero, then, wo Imvo a tianxforinor which di-poniJH iipon tbo I'otiitioti, hy ineio'.K of the rotiii y <'iirii'nt, of u tniiynolic dol] in- «iih! 11 iloMii] iron rinj; wiihoul polos, whil-*t iho conipound cur- lent hiippjioii (Vom tho ninehino excilon in tho soconilary colli fitnihr ciint'nt iinpiiUon IWIIowinj,' iiuhiinl ono iinotlior. The liuiiNloriiiulion coricNiioinlM lo liic iicl. >n llmt liikoH piiici' in » loiiiiy cmriiii dynnnic, oxcpt lli;il in liio tnm>ri)rnni' Ihu liulil iMttitoM abdiu llxoil coils, whil«t in Iho dyniimo tho iiiraiituie 18 mochanicully rotutod in r fixed Hold. In tliiH trunsfoimcr we liiivo three connocled currcniM dilVorini; liy \2i>° in jjliiwo, pro- diicin>{ 11 rotary liold, and this ha i lod to tlic idea that iho snino loss will occur that lakes jdacc in tho cuho of tho ciinentH in a niolor. This iicinally duos occur to tho oxtont of about 16 |)or cent losH in pressure, because tbo ratio of Irunsforination is not the ratio of tho number of turns on the primary lo those on the sooondury, but it is only ncco.ssary to inci'oaso tho number of turns in the secondary by 15 por cent in order to fully utilize the primary curronls. The olllcioncy of tho traiisforuior is conso- qiiontly onl} very slij^htly all'ectod. As a j^eiieral rule, tlio loss in the Iransformution and Iho ens' .A' tram, lot niers uie not of ffrcat iniporlanc'.', since it in generally proferable to build ma- chines for a relatively low voliagi>, and to transform tho energy to a high vollago before siijiplyin^r it to the conducting wires. In the case of very liigli jiressiiros, exceeding 1,<)U0 volts, it is much cheaper anil safer lo build a dynamo with a terminal volt- age of from M) to 100, and transform up to the required line polcniial, ll is, in fact, so meelf, and the low jirosure inachiiio works so much more eco- nomically that tho three or four per cent loss in tho transformer is balanced. When at tho second transformation at tho receiving station the currents aio lo lie used for running largo motors oi- lanijjs it will bo well to resolve tho combined curientM, for reasons similar to those which apply to dynamos. Fig. 11 shows ono of many methods of doing this. A, H, C ro- presont the long dislanco lead- carrying high prcssuies and small currents. The letters a, li, c, d rolbr to tho low pressui'o leads running directly to motors, etc. The tran.-.tinmer ring is shown dotted. The behavior of tho currents of the socondaiy winding is a rcproducling of that of those in the so|iaiato |iriniaiy cir- cuits, so that the legulation ol current iind tension is coinpura- tively simple. JJy connecting the secondary currents wo also secure the advantage that the long ami oxi)en.--ivu conductois need only bo three in.-tead ot timr in niiinbor. It can readily be ])roved that thf u-ei(/ht ot the. ('undwtor required Jor equal enenjy, is nut (jnutvrfor lliei /arc wires of (he rotary current system than /or the two wires of the ordinary system. In determin- ing iho seclion of the comiiictor for a coiniiiiicd rotary current, such as is used in transmitting jiower over a long distance, it is necessary to obtain a clear idea as to the amount of energy cou- voyetl. Tho iletcrminiitinri in tniulu in llio iMno way an in the ciwe ot'ati ni'ilinury altvniiilinf^ciirioiit, witli the aHNiimption that tlioi •) in (lo Holt'-iii(iiictii)n, i.o., no dillfi onco of jiliaNit holwron cur- rent ami t)lo('iri>nii>livy torco. Hliimlil Hclt'incliK'tion uxint, tlio Hnmc iiKMlihcaiiiitiM lliiil occur with an altcriiatir,^; currotit uImo proHont tlioipc IvoH with a rotary ciirrciit. Th' three coicl-otoM Hhouicl 1)0 coiiiiJ jix'd an ciinally hiadod. ninco only tho full normal load ni'cd ho con-idorud in dtitur^ninin^ llio noi'tinn of thu conduc- tor. Aw ihc lantof thortc thi'eo condition^ novor, or vory Huldoni, lioldN f{ood, tlicdotcrniinalion of tho sectional area of tho conductor would not 111' Hutllciuntly correct for |iractical work, though of coiii'Mo Iho ^onorai piopoHilion IioIiIh Iruu, In a cuntral Htation tho rotary currents would usually ho unconihino