Chapter 12 Magnetism 103 Copyright Goodheart-Willcox Co., Inc. Copyright Goodheart-Willcox Co., Inc. action. One common application is used in the automobile, where the solenoid is used to mesh the gear on the starting motor with the gear on the fl ywheel of the engine. At the same time, the solenoid action closes a switch that supplies current to the starting motor, Figure 12-14. Project 4—Door Chime and Project 8—Electric Engine in Chapter 22, Electrical/Electronic Projects, are based on the principles of the solenoid. You will fi nd them interesting and practical applications of magnetism. Hall Effect Devices Hall effect sensors are devices that produce output voltages in the presence of a magnetic fi eld. In most Hall effect devices, a strip of semiconductor material is exposed to a magnetic fi eld. A voltage is applied to opposite edges of the semiconductor. When the semiconductor material is then exposed to a magnetic fi eld, its output voltage (taken from the other two edges of the semiconductor) is in proportion to the magnetic fi eld. These sensors are used in various applications, such as measuring the speed of conveyor lines or counting the rotations of a shaft. See Figure 12-15. Magnetism by Induction Why will a magnet pick up a nail? Experimentation has revealed that if a piece of metal, such as a nail, is brought close to a magnet, it also becomes a magnet. It assumes a polarity that will cause attraction between the nail and the magnet, see Figure 12-16. If a second nail is brought close to the end of the fi rst nail, it will also be attracted, as in Figure 12-17. This is magnetism by induction, which is the basis of the theory of magnetism. Magnetic Shielding Closely related to the phenomenon of magnetism by induction is the problem of keeping a magnetic fi eld away from delicate testing instruments and meters. There is no known shield against magnetism. It passes through nonmagnetic materials as if they were not there. Try holding a piece of glass between a magnet and a nail. The nail is picked up as if the glass was not there. Iron core N S Goodheart-Willcox Publisher Figure 12-12. The iron core is drawn into the coil. Iron core N S Goodheart-Willcox Publisher Figure 12-13. The movement of the core stops when the magnetic forces are balanced. To gear Motor control Switch To switch Pivot To switch Goodheart-Willcox Publisher Figure 12-14. The solenoid is used to close the motor switch and to engage the gears in the automotive starter.