624 THE POPULAR SCIENCE MONTHLY. 



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 paral- 

 lel at, say, two thousand volts, may have a portion of their cur- 

 rent taken from them at that voltage for use in the immediate 

 neighborhood and the rest transformed up for distant transmis- 

 sion. 



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 stor- 

 age-battery operation direct. Second, the smaller weight of cop- 

 per 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 de- 

 vised 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 hav- 

 ing 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 estab- 

 lishments. 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 alter- 

 nation. 



The number of pulsations of commercial alternating currents 

 is usually over one hundred per second and is frequently double 



