Chapter 12 Magnetism 101 Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. If the core, upon which the coil is wound, is made of iron or low-carbon steel, the magnetic effect almost disappears when the current ceases to fl ow. Such magnetism remaining in the core is termed residual magnetism. In order to use the coil as a controlled electromagnet, you would need to wind it on a core that retains little of its magnetism after the magnetic fi eld has been removed. Another principle we should consider is the strength of the electromagnet. It depends, of course, on the ampere-turns and the permeability of the core. Electromagnets can be made to operate low-energy devices such as a doorbell, or devices requiring enough energy to move tons of weight as in a recycling center. Relays There are hundreds of electromagnetic applications in relays. A relay is a magnetic switch, and they are usually used when it is necessary to control a rather large electric current with a small control current. See Figure 12-9. Project 5—Magnetic Relay, in Chapter 22, is a good exercise to better understand the principles of controlling circuits by electromagnetism. Study Figure 12-9A, which illustrates a control circuit used in a relay coil. The coil is fi xed onto an iron core to enhance the magnetic fi eld. The control circuit is normally connected to a low-voltage (12–24 V), low-current (approximately 100–500 mA) power source. When current is permitted to fl ow in the control circuit, a magnetic fi eld is established. This magnetic fi eld then causes the contacts A and B to close. Usually, one contact is fi xed in place (B in this example), while the other contact (A) is on a movable armature. The armature moves in relation to the magnetic fi eld. If there is a magnetic fi eld present, the armature moves to the coil and closes the contacts, A and B. This completes a high-voltage, high-current circuit. Once the magnetic fi eld is removed, a tiny spring moves the armature away from the coil. Thus, a low-voltage, low-current circuit is able to control a much higher voltage and current circuit. Hans Christian Ørsted (1777–1851) A Danish physicist, Ørsted was the fi rst to discover that electrical current did indeed produce a magnetic fi eld. He studied at the University of Copenhagen and later taught there as a professor of physics. His efforts related to electricity and magnetism led many others into research on this topic. Scientist Profi le A B Controlled circuit B A Control circuit Coil Moving armature Spring Pivot point Goodheart-Willcox Publisher Mihancea Petru/Shutterstock.com Figure 12-9. A—A schematic diagram of a relay. B—An example of a relay with the protective plastic cover removed.