6i6 



NA TURE 



[October 29, 



dynamo, attached to the motor to be started, a portion of 

 the direct current that is being produced by one of the 

 running dynamos. This will cause the stationary direct 

 current dynamo to start running as a motor, and when 

 the right speed has been attained — that is, when the motor 

 is in step with the distant alternate current dynamo — the 

 alternate current can be switched on to the alternate 

 current motor. 



Actual plans are being seriously got out at the present 

 time, for using this exact method to transmit 5000 horse- 

 power over forty miles in Tasmania, the received power 

 being transformed by ten such combinations as are seen 

 in Fig. II, each of 500 horse-power. 



This subdivision of the machinery at the receiving 

 end, if accompanied by a similar subdivision of the 

 generating plant at the sending end of the line, will have 

 another most important advantage, viz. that a breakdown 

 of a dynamo or of a motor will not cause a stoppage in 

 the supply of power. A. factory is, no doubt, worked at 

 present with a single large engine ; the propulsion of a 

 steamer depends on the turning of a single powerful 

 screw ; but neither the unexpected stoppage of the factory 

 engine for say half-an-hour once every two or three months, 

 nor the delay of an Atlantic liner in mid-ocean for the 

 same time once in every half-dozen voyages, would 

 necessarily mean ruin. Were, however, the 10,000 

 horse- power dynamo at Deptford to be ever finished and 

 worked at its full output, it would be necessary, in order 

 to avoid a temporary hitch leading to the turning off 

 the current from many thousands of glow lamps, and 

 the plunging of a neighbourhood into darknes-:, to 

 always have dynamos of a capacity of 10,000 horse- 

 power kept idle in reserve. 



Experience has shown that the size of each dynamo in a 

 central station should be something like one-tenth of the 

 maximum output, and that it is sufficient to keep one, 

 or at the most two such dynamos, as a reserve, to pre- 

 vent temporary breakdowns interfering with the steady 

 supply of current. Until, then, a single centr.il station 

 is lighting some 500,000 glow lamps— or more than ten 

 times the total number at present attached to the 

 mains of the London Electric Supply Corporation — no 

 one but the Brunei of electricity would have had the 

 courage to embark on a 10,000 horse-power machine. 



At any rate, when during the next year or two it is re- 

 quired to transmit a large amount of power over a con- 

 siderable distance, it is probable that several alternate 

 current synchronizing motors, each coupled to a direct 

 current dynamo, will be employed at the receiving end of 

 the line. 



In cases, however, where there already exists an ex- 

 tended system of distributing alternate currents for electric 

 light, the introduction of motors into small workshops 

 and private houses will hardly be possible, unless the 

 motors can be made self-starting. Mr. Zipernowski's 

 motors, employed for driving the tools in a c irpenter's 

 shop at the Frankfort Exhibition, have been made self- 

 starting, and also fairly efficient, by adopting a com- 

 promise between the simple direct current motor, which 

 is self-starting but inefficient when used with alternating 

 currents, and the alternate current synchronizing motor, 

 which is efficient but not self-starting. 



The device employed by Mr. Zipernovvski, and which is 

 based on a communication made by Prof. G. Forbes to the 

 Royal Society of Edinburgh some eight years ago, is as 

 follows : — Send the alternating current round the field 

 magnet as well as round the armature of an alternate 

 current motor (Fig. 10), and attach a commutator to the 

 armature so as to reverse the current flowing round the 

 field magnet every time the armature coils Aj, A2, A3 pass 

 the field magnet coils Mj, M^, M3. On sending the alter- 

 nate current round such a motor, the motor will start, but 

 since at first the rapidity of alternation of the current will 

 be far greater than the rapidity of commutation there 



NO. I 148. VOL. 44] 



will be much sparking at the commutator and waste of 

 power. As, however, the armature turns more and 

 more qu'ckly, the commutation will be effected more 

 and more rapidly, until at last the armature will attain 

 such a speed that every tirrie the current is re- 

 versed by the distant dynamo the portion of the 

 current flowing round the field magnet of the motor 

 will be commutated by the rapidly rotating armature. 

 Hence the current flowing round this field magnet will 

 now be alwa\s in the same direction. But as it will not 

 be always of the same strength there will be more waste 

 of power than with a simple synchronizing motor. 



Such an arrangement as that adopted by Mr. 

 Zipernowski, then, furnishes a motor which, although not 

 as efficient and powerful for its weight as the synchro- 

 nizing motor previously described, has the advantage of 

 synchronizing fairly well, of being self-starting, and of 

 giving far betterresults than a direct current motor with 

 laminated field magnets used with alternating currents. 



It is possible, however, as proved by Prof. Ferraris in 

 1885, to design an alternate current motor on totally 

 different principles, and to construct a machine which 

 will work not merely without a commutator, but without 

 even any sort of rubbing contact. So that, in fact, the 



i d .. ■• e f 



Fig. 12. — Rotating magnetic field produced by tvvo alternating currents. 



ends of all the wires on a Ferraris motor may be per- 

 manently soldered, and the motor left in the hands of a 

 person who knows how to oil a machine but who is quite 

 ignorant of the trimming and adjustment of the brushes 

 I of an ordinary direct current motor. 



j Round an iron ring are wound four coils, as seen in 

 I Fig. 12, and through the two distinct circuits are sent two 

 ! harmonic alternating currents having the same periodic 

 I time and maximum amplitude, but differing by 90^ in 

 ! phase. The ring will therefore receive two magnetiza- 

 I tions along two fixed diameters at right angles to one 

 j another, the two magnetizations alternating approximately 

 ' according to the sine function of the time, and differing 

 \ by 90° in phase. And the composition of these two 

 \ magnetizations will give a " rotating magnetic Jield^^ which 

 ; will make one complete rotation in the periodic time of 

 alternation of the current. 



Six values of these tvvo currents are indicated in Fig. 

 12, the currents in a, c, and ^, being of their maximum 

 value in coils I, I,, and nought in coils II, II2 ; while in 

 b, d, and /, the currents in the four coils are equal, 



being each —of the maximum value. The arrow indi- 



cates the position which in each case would be taken up 



