WHAT MAKES THE TROLLEY CAR GO. 



421 



as it is a convenience but not a necessity. It can be seen at once 

 that whether it is used or not, the amount of power required to 

 operate the roads will be the same, but it is thought by most rail- 

 road managers that it is desirable, for then the relation between 

 the coal consumed and the power developed can be seen; and if 

 the showing is not as good as it should be, the engineer can remind 

 the firemen that they are not exercising as much care in feeding 

 the boilers as they should. Considered in this light, the watt- 

 meter acts as a check to wastefulness on the part of the employees. 

 The instruments marked C C C C serve the same purpose in 

 connection with the generators as the safety fuses do with respect 

 to the car motors; they are electro-magnetic devices used to open 



Fig. 24. -^Electric Kailway Switchboard. 



the generator circuits whenever the current reaches a strength that 

 is sufiicient to injure the machine. These devices are called circuit 

 breakers. As will be noticed, there are four located directly above 

 the four a switches, and, at the farther end of the board, a large 

 number located directly above the b switches. The latter act to open 

 the individual circuits when the currents flowing in them become 

 too strong, and the former are controlled entirely by the current of 

 the generator circuits. A circuit breaker is more reliable than a 

 safety fuse, because it acts quicker. With the fuse the current 

 must act for some time before it can melt the metal, as a sufiicient 

 amount of heat can not be generated instantly. "With the circuit 

 breaker, however, the action -is instantaneous, for as soon as the 

 current reaches the predetermined strength the magnetism of the 



