ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
stones were products of nature and not the result of the efforts
of man. The Greeks called these substances magnetite.
The Chinese are said to have been aware of some of the effects
of magnetism as early as 2600 B.C. They observed that stones
similar to magnetite, when freely suspended, had a tendency to
assume a nearly north and south direction. Because of the
directional quality of these stones, they were later referred to
as lodestones or leading stones.
Natural magnets, which presently can be found in the United
States, Norway, and Sweden, no longer have any practical use as
it is now possible to easily produce more powerful magnets.
(3) Artificial Magnets. Magnets produced from magnetic
materials are called artificial magnets. They can be made in a
variety of shapes and sizes and are used extensively in
electrical apparatus. Artificial magnets are generally made
from special iron or steel alloys which are usually magnetized
electrically. The material to be magnetized is inserted into a
coil of insulated wire and a heavy flow of electrons is passed
through the wire. Magnets can also he produced by stroking a
magnetic material with magnetite, or with another artificial
magnet. The forces causing magnetization are represented by
magnetic lines of force, very similar in nature to the
electrostatic lines of force.
Artificial magnets are usually classified as permanent or
temporary, depending on their ability to retain their magnetic
properties after the magnetizing forces have been removed.
Magnets made from substances, such as hardened steel and certain
alloys which retain a great deal of their magnetism, are called
permanent magnets. These materials are relatively difficult to
magnetize because of the opposition offered to the magnetic
lines of force as the lines of force try to distribute
themselves throughout the material. The opposition that a
material offers to the magnetic lines of force is called
reluctance. All permanent magnets are produced from materials
having a high reluctance.
A material with a low reluctance, such as soft iron or annealed
silicon steel, is relatively easy to magnetize but will retain
only a small part of its