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Chapter Four
MAGNETISM Historical Facts Magnetism was first discovered in a natural rock called lodestone. In the 1600s, Sir William Gilbert discovered that the earth was a great magnet with north and south poles. Gilbert shaped a piece of lodestone into a sphere and demonstrated that a small compass placed at any spot on the sphere would always point, as it does on the earth, toward the north pole. Lodestone is also known as magnetite.
Lodestones If a lodestone is suspended from a string, one end will always rotate and point toward the north pole of the earth. See Figure 4-1. Lodestone is one material where the molecules can be aligned with the magnetic field of the earth. Most materials contain molecules that are arranged randomly and cannot be made to align. Some material, such as iron, nickel, and cobalt, can be made magnetic by exposing them to a magnetic field. While some materials will return the magnetism after they have been magnetized, many materials will lose this property.
Polarity All magnets have a polarity. If a permanent magnet were to be cut or broken, the result is two
Figure 4-1. (lodestone).
A freely suspended natural magnet
magnets, each with their own polarity. See Figure 4-2. When a magnet is freely suspended, the poles tend to point toward the north and south magnetic poles of the earth, which led to development of the compass. Magnetism provides a link between mechanical energy and electricity. By the use of magnetism, an automotive generator converts some of the mechanical power developed by the engine to electromotive potential (EMF). Going the other direction, magnetism allows a starter motor to convert electrical energy from the battery into mechanical power to crank the engine. A magnet can be any object or device that attracts iron, steel, and other magnetic materials. There are three basic types of magnets as follows: • Natural magnets • Man-made magnets • Electromagnets
Reluctance Magnetic lines of force are transmitted more easily through a metallic substance rather than air. Reluctance is the term used to describe the resistance to the movement of magnetic lines of force and permeability is the term used to list the relative reluctance of a material. For example, steel is a better conductor of magnetic lines of force than air and is given a permeability factor number of one compared to some alloys whose permeability is over 2,000 and can be as high as 50,000.
Figure 4-2. Magnetic poles behave like electrically charged particles.
