The programme clocks which were examined were self-winding clocks, but were connected by wires to the master clock which corrected them each hour. Each time the long hand of the master clock came to twelve it closed an electric circuit through all the clocks in the system. In each clock the current passed around an electro-magnet and caused it to pull an armature against a metal stop and set each long hand exactly at twelve. This master clock is sometimes situated many miles away and may correct the time for a whole city. Thus a master clock at Washington, D. C., furnishes standard time to all parts of the United States. The master clock which we examined also closed the circuit at proper intervals through a series of programme bells placed in the various class rooms, and these called and dismissed classes automatically.
Fig. 45
15. Watchman's Time Detector ([Fig. 45]).—This is a device to compel a watchman to make his appointed trips. Push buttons or switches are distributed about the building at various points, and it is made his duty to close the circuits at these points at stated times. When he does so, the fact is recorded by electro-magnets puncturing, or, in some way, marking a revolving time card in the clock.
Fig. 46
16. Circuit Breakers ([Fig. 46]).—Electro-magnets are used to open switches and thus protect dynamos and other machines against a larger electric current than they are able to carry. The switch is held closed by a spring which, by an adjusting device, may be tightened or loosened. A dynamo which we examined had its circuit breaker adjusted so that it would remain closed if any current under 1500 amperes passed, but if a greater current than that passed it would strengthen the magnet sufficiently to open the switch and thus break the circuit.
