ELECTRICITY AT THE WORLD'S FAIR. 739 



other hand, it be supplied with direct currents, it will also run 

 as a motor, and deliver multiphase alternating currents. This 

 apparatus promises to hold an important place, if not an in- 

 dispensable one, in any complete system of electric distribu- 

 tion. For many purposes, such as electroplating and electro- 

 typing and all forms of electro-decomposition, the continuous 

 current is essential, and for other uses, in the present state of the 

 art, it can not well be dispensed with. One of these uses is the 

 operation of electric railways. The alternating-current motor, 

 though answering many of the requirements of a commercial 

 motor, has one disadvantage in comparison with the motor driven 

 by direct or continuous currents. It has a less powerful starting 

 torque that is, the pull upon the armature tending to rotate it is 

 much less at the start than in the direct-current machine. In 

 railway work a powerful starting torque is of the greatest impor- 

 tance, as a motor is frequently called upon to exert four or five 

 times the power in starting that is needed to keep the cars in mo- 

 tion. Whether the direct-current motor will continue to be essen- 

 tial for railway work or not, it is evident that a device which 

 enables either direct or alternating currents to be supplied to the 

 consumer at will must add much to the flexibility and complete- 

 ness of any system of distribution. With the apparatus as at 

 present worked out it is possible to place a generating dynamo at 

 the source of power, say a waterfall twenty miles away, produce 

 with this multiphase alternating currents, raise the potential of 

 these to any desired amount by means of a step-up converter, 

 pass them through the line, and then at the distribution end re- 

 duce them through the medium of a step-down converter to any 

 suitable pressure. These reduced currents may then be used 

 direct for operating alternating-current motors, for running in- 

 candescent or arc lamps, and, through the medium of the rotary 

 transformer, direct currents may be obtained for operating street 

 railways and other continuous-current motors, both classes of 

 lights, and all kinds of chemical decomposition apparatus. It 

 might be supposed that the multiphase system of alternating cur- 

 rents was a departure away from the direction of line economy, 

 so necessary a consideration in long-distance transmission, since 

 this system requires two or more circuits. This, however, is not 

 the case. It was early discovered by Mr. Tesla that the multiple 

 circuits could have a common return wire, and it appears that 

 the amount of copper in the combined circuits is actually less 

 than in the single circuit required for the ordinary single-phase 

 current. 



The value of the departure in alternating apparatus made by 

 Mr. Tesla has been very generally appreciated in the electrical 

 world, and electric companies, both in this country and abroad, 



