Page:Popular Science Monthly Volume 43.djvu/759

Rh 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 indispensable one, in any complete system of electric distribution. For many purposes, such as electroplating and electrotyping 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 importance, 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 motion. Whether the direct-current motor will continue to be essential 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 completeness 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 reduce 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 currents 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 departure in alternating apparatus made by Mr. Tesa has been very generally appreciated in the electrical world, and electric companies, both in this country and abroad