The solenoid
A cylindrical coil, having a movable ferromagnetic core, can be useful for various things. This is a solenoid. Electrical relays, bell ringers, electric "hammers," and other mechanical devices make use of the principle of the solenoid.
A ringer device
Figure 8-7 is a simplified diagram of a bell ringer. Its solenoid is an electromagnet, except that the core is not completely solid, but has a hole going along its axis. The coil has several layers, but the wire is always wound in the same direction, so that the electromagnet is quite powerful. A movable steel rod runs through the hole in the electromagnet core.
When there is no current flowing in the coil, the steel rod is held down by the force of gravity. But when a pulse of current passes through the coil, the rod is pulled forcibly upward so that it strikes the ringer plate. This plate is like one of the plates in a xylophone. The current pulse is short, so that the steel rod falls back down again to its resting position, allowing the plate to reverberate: Gonggg! Some office telephones are equipped with ringers that produce this noise, rather than the conventional ringing or electronic bleeping ernitted by most phone sets.
A relay
In some electronic devices, it is inconvenient to place a switch exactly where it should be. For example, you might want to switch a communications line from one branch to another from a long distance away. In many radio transmitters, the wiring carries high-frequency alternating currents that must be kept within certain parts of the circuit, and not routed out to the front panel for switching. A relay makes use of a solenoid to allow remote-control switching.
A diagram of a relay is shown in Fig. 8-8. The movable lever, called the armature, is held to one side by a spring when there is no current flowing through the electromagnet. Under these conditions, terminal X is connected to Y, but not to Z. When a sufficient current is applied, the armature is pulled over to the other side. This disconnects terminal X from terminal Y, and connects X to Z.
There are numerous types of relays used for different purposes. Some are meant for use with dc, and others are for ac; a few will work with either type of current. A normally closed relay completes the circuit when there is no current flowing in its electromagnet, and breaks the circuit when current flows. A normally open relay is just the opposite. ("Normal" in this sense means no current in the coil.) The relay in the illustration (Fig. 8-8) can be used either as a normally open or normally closed relay, depending on which contacts are selected. It can also be used to switch a line between two different circuits.
Some relays have several sets of contacts. Some relays are meant to remain in one state (either with current or without) for a long time, while others are meant to switch several times per second. The fastest relays work dozens of times per second. These are used for such purposes as keying radio transmitters in Morse code or radioteletype.
The dc motor
Magnetic fields can produce considerable mechanical forces. These forces can be harnessed to do work. The device that converts direct-current energy into rotating mechanical energy is a dc motor.
Motors can be microscopic in size, or as big as a house. Some tiny motors are being considered for use in medical devices that can actually circulate in the bloodstream or be installed in body organs. Others can pull a train at freeway speeds. In a dc motor, the source of electricity is connected to a set of coils, producing magnetic fields. The attraction of opposite poles, and the repulsion of like poles, is switched in such a way that a constant torque, or rotational force, results. The greater the current that flows in the coils, the stronger the torque, and the more electrical energy is needed.
About the Author
Stan Gibilisco is one of McGraw-Hill's most prolific and popular authors, specializing in electronics and science topics. His clear, reader-friendly writing style makes his science books accessible to a wide audience, and his background in research makes him an ideal editor for professional references and course materials. He is the author of The Encyclopedia of Electronics; The McGraw-Hill Encyclopedia of Personal Computing; and several titles in the popular Demystified library of home-schooling and self-teaching books. His published works have won numerous awards. The Encyclopedia of Electronics was chosen a "Best Reference Book of the 1980s" by the American Library Association, which also named his McGraw-Hill Encyclopedia of Personal Computing a "Best Reference of 1996." Stan Gibilisco maintains a Web site at www.sciencewriter.net.
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