Circuit Failures

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Circuit Configurations

Construction Mechanic Basic Volume 2

Electromagnetism

CIRCUIT FAILURES
Open circuit (fig. 1-14, view A). An open circuit is a break or interruption in the circuit, such as a wire that has come loose or a slipped connection that is not making contact. But the expression of an open circuit is not only used when wire connections are actually separated as in a switch but also when the resistance in the wiring circuit is so that no current can flow between the battery and the unit it operates. A good example of such a condition is rust and corrosion that forms and accumulates at a battery cable or terminal.

Ground circuit. A ground circuit occurs when any part of the wiring circuit is touching the vehicle frame inadvertently. A ground involves accidental or unintentional contact between copper and the iron frame.

MAGNETISM
Learning Objective: Describe the theory of magnetism and the principles of electromagnetism and electromagnetic induction.

Magnetic field is described as invisible lines of force which come out of the North Pole and enter the South Pole. For example, if iron filings were sprinkled on a piece of glass on top of a bar magnet, the filings would form themselves in curved lines

Figure 1-14.-( A) Open circuit; (B) Short circuit.

(fig. 1-15). These curved lines, extend from the two poles of the magnet, follow the magnetic lines of force surrounding the magnet. Lines of force rules are as follows:

The lines of force (outside the magnet) pass from the North Pole to the South Pole of the magnet.

The lines of force act somewhat as rubber bands and try to shorten to minimum length.

The lines of force repel each other along their entire length and try to push each other apart.

The rubber band characteristic opposes the push-apart characteristic.

The lines of force never cross each other. The magnetic lines of force, taken together, are referred to as the magnetic field of the magnet.

The magnetic fields of a bar and of a horseshoe magnet are shown in figure 1-16. In each, note how the lines of force curve and pass from the North Pole to the South Pole.

Effects between magnetic poles (fig. 1-17). When two UNLIKE magnetic poles are brought together, they attract. But when LIKE magnetic poles are brought together, they repel. These actions can be explained in terms of the rubber band and the push-apart characteristics. When unlike poles are brought close to each other, the magnetic lines of force pass from the North Pole to the South Pole. They try to shorten (like rubber bands) and, therefore, try to pull the two poles together. On the other hand, if like poles are brought close to each other, lines of force going in the same direction are brought near each other. Because these lines of force attempt to push apart, a repelling effect results between the like poles.