Page:Popular Science Monthly Volume 45.djvu/644

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advantage which the alternating current has over the direct, and that is what may be termed the "flexibility," commercially, of the former. The alternating-current machines operated in parallel at, say, two thousand volts, may have a portion of their current taken from them at that voltage for use in the immediate neighborhood and the rest transformed up for distant transmission.

The advantages of the direct-current system would be two: First, the simpler methods of motor operation by its means, and the availability of the current for electrochemical work and storage-battery operation direct. Second, the smaller weight of copper necessary on the transmitting wire, for the three reasons of the evenness of flow, the absence of self-induction on the line, and the absence of skin resistance in direct-current transmission. The effects of the two latter phenomena will be discussed later.

Inventive effort has, singularly, stayed in the rut of work on alternating-current transmission, and in attempting anything on the scale of the Niagara Falls undertaking it would be perilous, even had it been considered for other reasons advisable, to depart more than necessary from usual practice.

Lately, and particularly owing to the brilliant work of a young man, a native of Smiljan Lika, a border country of Austria-Hungary, by name Nikola Tesla, there have been devised forms of apparatus, generating as well as consuming, by means of which alternating currents may be economically used for operating motors. To express it very roughly, his method amounts to arranging an armature within a magnetic ring and causing opposite magnetic poles to revolve around the ring so as to cause rotation of the armature.

The operation of these devices is preferably by means of a polyphase alternating current—that is, a flow of electricity having more than one pulsating current.

Before finally deciding on what system of transmission to use, the Cataract Construction Company asked for plans for a system for the purpose from a number of electrical engineering establishments. Twenty-four distinct ones were submitted, more than one of the tendering companies having sent several different plans to be chosen from. No individual one was, however, accepted in toto, but instead a design was adopted embodying such points of value as could be assembled in one suitable type of machine, and the Westinghouse Company received the contract for it. The system on which the generators work is the Tesla two-phase, and is notably peculiar on account of the low periodicity of alternation.

The number of pulsations of commercial alternating currents is usually over one hundred per second and is frequently double