2i8 THE POPULAR SCIENCE MONTHLY 



type. The value of alternating currents was not appreciated, however, 

 as the principles of transformation and their value for transmission 

 were not understood. The high degree of perfection to which the 

 direct-current motor has been developed naturally led to its use for 

 railways as city lines were extended, and this tendency has resulted in 

 the enormous mileage of suburban and interurban electric railways. 

 This tendency was aided by the development of the rotary converter 

 which permitted the transmission of power by alternating currents and 

 its conversion to direct current for supplying the trains. The direct 

 current motor operates at about 600 volts and thus fixes the value of 

 the voltage on the trolley or third rail. The voltage being fixed, the 

 total energy at a car is proportional to the current. Thus, for light 

 cars and infrequent traffic, the loss in the trolley and feed wires, due 

 to the passage of current delivering energy to the car, is sufficiently 

 small to allow operation at comparatively long distances from the point 

 of generation. With increasing size of cars or trains the points of 

 supply must be brought nearer together and be made of greater ca- 

 pacity. In the case of the New York Central installation, the average 

 distance between such stations is four miles. Each of these sub- 

 stations contains transformers for reducing the voltage from the trans- 

 mission line and rotary converters for changing the alternating to 

 direct current. The amount of current taken by a train on this system 

 may go to very large values and this necessitates large trolley and feed- 

 ing conductors when the traffic increases in volume. Under this system 

 the feeding conductor may be either trolley or third rail and the col- 

 lecting device, the trolley wheel or third rail shoe. The latter must be 

 used when the currents are of large value. 



With increasing length of line the cost of sub-stations and feeders 

 in the direct current system becomes prohibitive. This has lead to the 

 development of alternating current systems, in which the energy is 

 generated, transmitted and delivered to the car at voltages as high as 

 15,000, with consequent reduction in values of the necessary current. 

 Theoretically, the reduction in the size of conductors necessary to carry 

 the current is inversely proportional to the square of the voltage. 

 While certain characteristics of the alternating current system reduce 

 this theoretical value quite materially, the gain in this respect is never- 

 theless enormous, and the distance between feeding points increases to 

 between thirty and fifty miles, depending on the density of the traffic. 

 The trolley wire alone is often the only feeding conductor required. 

 Further, the apparatus in the sub-stations in these systems comprises 

 stationary transformers only which require no attendance. Two con- 

 spicuous alternating current systems for railway operation have been 

 developed. The single phase system, which has been almost the only 

 alternating current system used in this country, and the three-phase 

 system, which has met with some favor in Europe. 



