The instrument marked _W_, located between the _a_ switches, is called a wattmeter, and its office is to indicate the amount of power furnished by the generators. This instrument is not always used, 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 railroad 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. [Illustration: FIG. 24.--ELECTRIC RAILWAY SWITCHBOARD.] 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 the generator circuits whenever the current reaches a strength that is sufficient 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 sufficient 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 operative parts of the device becomes sufficiently strong to cause it to act. A circuit breaker is simply a switch that is arranged to be opened automatically by the action of a magnet, instead of by the hand of the operator. The switch part of the apparatus is held in place by a catch that is set much after the fashion of the catch in a mouse trap--that is, so that the least pressure will disengage it. A strong spring acts to throw the switch open, and as soon as the catch is tripped by the actuating magnet the force of the spring comes into action and the circuit is opened.