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 wattmeter 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
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