An iron core, surrounded by a coil of wire is called an
electromagnet. Such a magnet is a magnet only when current
flows through the coil. When the current is stopped, the iron
core returns almost to its natural state. Electromagnets are
used for a variety of purposes in radio, such as for testing
buzzers, headphones, generators, motors, relays, etc.
A compass consists of a small magnetized piece of steel
(the magnet needle) mounted on a vertical pivot which is enclosed
in a brass box with a glass top so as to prevent air currents
from interfering with the position taken by the needle.
If the compass needle is allowed to move freely, one end of
the needle will point toward the North of the earth, with the axis
of the needle in a North and South line. Any magnet which is
free to move will do the same thing.
The end pointing toward the North is called the “North
Seeking Pole” and the other end the “South Seeking Pole.” It
is more common to call them North Pole and South Pole respec-
tively. Some unseen force, therefore, must be drawing the poles
of the magnet for it to turn on the pivot or to keep it pointing
North and South. This force is the magnetic force of the earth.
Now if we take a bar of steel which has been magnetized and
place one end close to the magnet needle, the effects of a new
magnetic force will be noticed; that will be the force exerted
by the magnetized bar of steel.
When the N pole of the magnetized bar of steel is placed
near the N pole of the compass needle it will be noticed that
they repel each other. If the S pole of the compass needle is
approached with the S pole of the magnetized bar, again the
needle will be repelled. These are like poles (similar). If unlike
poles are approached they attract. (See Fig. 5.) It is importent
to realize that the “magnetic force” is always associated
with a magnet, whether the magnet is acting on a compass, piece
of steel, iron or not--or put it this way--whether the magnet
is working or not. The space over which this magnetic force
extends is called the magnetic field. And now we want to show
that it is easy to imagine the force in the field as extending from
the magnet in lines, called “lines of force.”
If a small compass needle is placed anywhere near a bar
magnet, the needle takes a definite position.
If a bar magnet is placed under a sheet of paper and then
the compass needle is put down at a number of positions and
