There are four kinds of electric-tramway traction which, though apparently rather puzzling, are readily explained. These are the Conduit; the Surface Contact; the Overhead (or trolley); in each of which the current is conveyed to the line—as in an electric railway—from a power house; and the Accumulator, or Self-contained Car, the motive power being obtained from storage batteries carried on the car itself, and these supply the current direct to the motor on the car.
FIG. 15. TRAM-CAR IN PARIS EQUIPPED FOR COMBINED OVERHEAD TROLLEY AND SURFACE CONTACT SYSTEM
By permission of the Dolter Electric Traction Co., Ltd., London
In the conduit system the main conductors (or feed-wire), always in this country placed underground, are carried in a conduit or tube under the track, which has a narrow longitudinal slit on its upper surface level with the road. Through this slit passes a bracket carried by the car in such a manner as to make contact with the two conduit-conductors. The objections to this system are the heavy cost of construction, its liability to derangement from floods, the expense of cleaning the conduits, and its tendency to accumulate filth.
The closed conduit, or surface contact system, consists of a series of plates or studs placed along the track a few feet apart and flush with the road, and insulated from each other. Under ordinary circumstances these are disconnected with the conductor, which is laid entirely below the surface, but when a car passes over them they become, by means of switches, automatically connected with it, so that the current can be conveyed through them to the car motors. In other words, the studs are “alive” while the car is over them, and “dead” as soon as it has passed. This is a very practicable method, and in certain cases is preferable to the open conduit. Defects, however, there are, but the Dolter apparatus claims to have overcome them, and it is greatly in its favour that the system has been successfully worked in Paris for more than two years. It has the merit of readily lending itself to a combination with the overhead trolley system.
Of all systems, by far the best known to the public is that of “overhead,” recognised immediately by the tall iron poles inseparable from its adoption. Ninety-five per cent. of the world’s electric tramways are worked on the overhead principle. The distribution of electric energy is by means of a wire, called the trolley wire, upheld by insulated brackets on poles twenty feet above the ground, along the entire track, which is divided into sections, each section taking its current from the main conductor-wire, which is laid underground, through the iron poles. Should any one section of the trolley wires meet with mishap, only the cars working on that section are stopped; those on the remaining divisions, having an independent source of current, continue to run
