PLATE A.-HOOD 4L4812 - Caterpillar

Illustration 4 g01069093
Because such flux line are circular, the magnetic field has no north pole or no south pole. Individual circular fields merge when the wire is wound into a coil. The result is a unified magnetic field with north and south poles, as shown in Illustration 5.
Illustration 5 g01069096
As long as current flows through the wire, it behaves just like a bar magnet. The electromagnetic field remains as long as current flows through the wire. The field that is produced on a straight wire does not have enough magnetism in order to do work. To strengthen the electromagnetic field, the wire can be formed into a coil. The magnetic strength of an electromagnet is proportional to the number of turns of wire in the coil, and the current flowing through the wire. Whenever electrical current flows through the coil of wire, a magnetic field, or lines of force, build up around the coil. If the coils are wound around a metal core, like iron, the magnetic force strengthens considerably.Relays and Solenoids
Types of electromagnets that are typically used in Caterpillar machines are relays and solenoids. Relays and solenoids operate on the electromagnetic principle, but function differently. Relays are used as an electrically controlled switch. A relay is made up of an electromagnetic coil, a set of contacts, and an armature. The armature is a movable device that allows the contacts to open and to close. Illustration 6 shows the typical components of a relay.
Illustration 6 g01069098
When a small amount of electrical current flows in the coil circuit, the electromagnetic force causes the relay contacts to close. This process provides a much larger current path to operate another component, such as, a starter. A solenoid is another device that uses electromagnetism. Like a relay, the solenoid also has a coil. Illustration 7 shows a typical solenoid. When current flows through the coil, electromagnetism pushes or pulls the core into the coil creating linear, or back and forth movements. Solenoids are used to engage starter motors, or control shifts in an automatic transmission.
Illustration 7 g01069099
Electromagnetic Induction
Illustration 8 g01069100
The effect of creating a magnetic field with current has an opposite condition. It is also possible to create current with a magnetic field by inducing a voltage in the conductor. This process is known as electromagnetic induction. Electromagnetic induction happens when the flux lines of a magnetic field cut across a wire or any conductor. It does not matter whether the magnetic field moves or the wire moves. When there is relative motion between the wire and the magnetic field, a voltage is induced in the conductor. The induced voltage causes a current to flow. When the motion stops, the current stops.If a wire is passed through a magnetic field, voltage is induced. The voltage induced strengthens when the wire is wound into a coil. This method is the operating principle that is used in speed sensors, generators, and alternators. In some cases, the wire is