Copyright Goodheart-Willcox Co., Inc. 102 Electricity Copyright Goodheart-Willcox Co., Inc. Another similar application is drawn in Figure 12-10. Here the coil is in series with the contact points. This is one type of a circuit overcurrent protective device (a circuit breaker). As long as the current fl owing in the circuit is within safe limits, the magnetic force of the coil will not overcome the tension of the spring. If the current exceeds a specifi ed value, contact B is pulled down by its moving armature. This opens the circuit, because contacts A and B are now open. The device is arranged so that the moving armature that has contact point B on it must be manually reset before the normal operation of the circuit can be resumed. The Reed Relay The reed relay is another similar application of magnetism. Referring to Figure 12-11, notice that two magnetically sensitive switch contacts are enclosed in a glass tube. If a permanent magnet is brought close to the glass tube, it causes the switch contacts to close. The reed relay can also be operated by an electromagnet. The operating coil is placed around the reed relay. Once energized, the coil’s magnetic fi eld closes the circuit by moving the reeds (or contacts) of the switch. This type of device is often used in security systems to indicate when a door or window has opened or closed. Solenoids as Switches Solenoids, discussed earlier, can also be used to convert electricity to magnetism to mechanical motion, Figure 12-12. We have already learned that when an electrical current fl ows through a coil of wire, the coil becomes a magnet and has a north and south pole similar to a permanent magnet. This coil attracts particles of iron in much the same manner as a permanent magnet. If a movable core of soft iron is placed in the center of such a solenoid coil, it is attracted by the coil, Figure 12-12, and drawn into the center of the coil. The solenoid will actually pull the iron core into the coil. The core’s movement stops when the attracting forces of each end of the coil are balanced, Figure 12-13, and the core has centered itself in the coil, where the magnetic fi elds are the strongest. This type of device has many applications. The movement of the iron core can be mechanically linked to switches, levers, or gears to produce the desired mechanical A B B A Coil Moving armature Spring Pivot point Goodheart-Willcox Publisher Figure 12-10. A—A schematic diagram of the magnetic type of circuit breaker. B—A circuit breaker. Magnetic switch contacts Magnet causes switch to close Magnet Wire terminal Wire terminal Switch open Glass tube N S Goodheart-Willcox Publisher Figure 12-11. A sketch of a reed relay. The magnet causes the reed contacts to close.