9.
MAGNETISM
Magnetism plays a most important role in electricity and electronics.
Practically all electrical power is generated, distributed, and used through the
medium of magnetism. Power generators, transformers, and motors have magnetism as
their basis of operation.
The radio transmitter that develops a signal, the
antenna that radiates it, the antenna that receives it, and the radio receiver that
delivers the sound--these too function on the basis of magnetism. It is therefore
necessary that we understand some of the principles of magnetism if we want to
understand some of the principles of electricity and electronics.
a. Magnetic Field. If we were to take a bar magnet, lay a piece of glass on
top of it, and then sprinkle iron filings on the glass, we would find that the
filings would become arranged in curved lines as shown in A of figure 11. These
curved lines, extending from the two poles of the magnet (north and south), follow
the magnetic lines of force surrounding the magnet.
(Every magnet will have a
north and a south pole.) These lines of force are shown in B of figure 11. Note
that the concentration of iron filings is greatest at the ends of the magnet,
showing that the lines of force have their greatest concentration at these ends.
The magnetic lines of force are said to pass from the north (N) to the south (S)
pole, and return through the body of the magnet.
Figure 11.
Magnetic lines of force traced by iron filings.
b. Characteristics of Permanent Magnets. A permanent magnet is usually a bar
of steel that has been magnetized. This bar may be made into any convenient shape
to satisfy the needs of the device.
By bending the bar around in a horseshoe
shape, the lines of force can be concentrated in a smaller space than is the case
with a bar magnet.
Most useful magnets have some variation of a horseshoe
configuration.
(1) Like poles repel, and unlike poles attract. This can be proved by the
simple experiment using bar magnets as shown in figure 12. The magnets
will not touch when N and N poles are presented to each
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