Page:Popular Science Monthly Volume 43.djvu/748

728 limited space between the motor and road bed in which to make a satisfactory rail contact. The return path for the current is through the traffic rails and iron girders of the elevated structure, the rails being copper-banded at the joints and joined by bands of the same material to the girders. Feeder rails extend from the power house for three fifths of the length of the line and are cross connected to the supply rails at every rail joint. The train equipment of the road consists of eighteen trains, weighing when loaded about ninety-six tons each, the motor car accounting for thirty tons of this weight and the other cars for twenty-two tons each.

The central figure of the power-house equipment is the great two-thousand-horse-power generator from the shops of the General Electric Company, said to be the largest machine yet built. It occupies the middle space of the power house and is driven by an Allis-Corliss cross-compound engine, which is a duplicate of the one in the Westinghouse plant in Machinery Hall. It is a direct-current machine of what is known as the multipolar type. This is a type of machine which has been developed in recent years in response to the increasing demands of railway power and central lighting stations for larger units of power. In machines of the power desired slow speed becomes essential, and this requirement has resulted in radically transforming the design of the dynamos. The two-pole field magnet, common in all machines a few years back, has given place to a multipolar one, generally made in the form of a ring-shaped yoke with inwardly protruding pole pieces, though this construction has been reversed in some large generators constructed by Siemens, in which the field poles radiate from a central hub, and the armature, made in the form of a flattened ring or band, is placed on the outside, its outer surface constituting the commutator upon which the brushes bear. A fine example of this machine coupled direct to a thousand-horse-power triple-expansion upright engine is to be seen in Machinery Hall. In the intramural generator the field consists of two massive semicircles of cast steel, bolted together, the lower of which is provided with supporting feet. This yoke is fifteen feet in diameter and three broad and with its twelve poles weighs over forty tons. The armature is what is known as the ironclad type, and is ten feet and a half in diameter, and weighs complete about thirty-five tons. The ironclad type of armature now used upon all railway motors and large generators is a comparatively recent development, and possesses marked advantages both mechanically and electrically. Its characteristic feature is the imbedding of the coils in the laminated iron core, either by forming tubular passages through this core near the edge or making it with open slots narrowed at the mouth to securely hold the