7 z8 THE POPULAR SCIENCE MONTHLY. 



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 Gen- 

 eral 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 re- 

 quirement 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 protrud- 

 ing 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 com- 

 paratively 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 mak- 

 ing it with open slots narrowed at the mouth to securely hold the 



