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 capacity. In the case of the New York Central installation, the average distance between such stations is four miles. Each of these substations contains transformers for reducing the voltage from the transmission 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 feeding conductors when the traffic increases in volume. Under this system the feeding conductor may be either trolley or third rail and the collecting 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 nevertheless 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 conspicuous 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.