Electricity for Boys | Page 7

piece of steel in
which an electric force is exerted at all times. An electro-magnet is a
piece of iron which is magnetized by a winding of wire, and the magnet
is energized only while a current of electricity is passing through the
wire.
ELECTRO-MAGNET.--The electro-magnet, therefore, is the more
useful, because the pull of the magnet can be controlled by the current
which actuates it.
The electro-magnet is the most essential of all contrivances in the
operation and use of electricity. It is the piece of mechanism which
does the physical work of almost every electrical apparatus or machine.
It is the device which has the power to convert the unseen electric
current into motion which may be observed by the human eye. Without
it electricity would be a useless agent to man.

While the electro-magnet is, therefore, the form of device which is
almost wholly used, it is necessary, first, to understand the principles of
the permanent magnet.
MAGNETISM.--The curious force exerted by a magnet is called
magnetism, but its origin has never been explained. We know its
manifestations only, and laws have been formulated to explain its
various phases; how to make it more or less intense; how to make its
pull more effective; the shape and form of the magnet and the material
most useful in its construction.
[Illustration: Fig 5. PLAIN MAGNET BAR]
MATERIALS FOR MAGNETS.--Iron and steel are the best materials
for magnets. Some metals are non-magnetic, this applying to iron if
combined with manganese. Others, like sulphur, zinc, bismuth,
antimony, gold, silver and copper, not only are non-magnetic, but they
are actually repelled by magnetism. They are called the diamagnetics.
NON-MAGNETIC MATERIALS.--Any non-magnetic body in the
path of a magnetic force does not screen or diminish its action, whereas
a magnetic substance will.
In Fig. 5 we show the simplest form of magnet, merely a bar of steel (A)
with the magnetic lines of force passing from end to end. It will be
understood that these lines extend out on all sides, and not only along
two sides, as shown in the drawing. The object is to explain clearly
how the lines run.
[Illustration: Fig. 6. SEVERED MAGNET]
ACTION OF A SEVERED MAGNET.--Now, let us suppose that we
sever this bar in the middle, as in Fig. 6, or at any other point between
the ends. In this case each part becomes a perfect magnet, and a new
north pole (N) and a new south pole (S) are made, so that the
movement of the magnetic lines of force are still in the same direction
in each--that is, the current flows from the north pole to the south pole.

WHAT NORTH AND SOUTH POLES MEAN.--If these two parts are
placed close together they will attract each other. But if, on the other
hand, one of the pieces is reversed, as in Fig. 7, they will repel each
other. From this comes the statement that likes repel and unlikes attract
each other.
REPULSION AND ATTRACTION.--This physical act of repulsion
and attraction is made use of in motors, as we shall see hereinafter.
It will be well to bear in mind that in treating of electricity the north
pole is always associated with the plus sign (+) and the south pole with
the minus sign (-). Or the N sign is positive and the S sign negative
electricity.
[Illustration: Fig. 7. REVERSED MAGNETS]
POSITIVES AND NEGATIVES.--There is really no difference
between positive and negative electricity, so called, but the foregoing
method merely serves as a means of identifying or classifying the
opposite ends of a magnet or of a wire.
MAGNETIC LINES OF FORCE.--It will be noticed that the magnetic
lines of force pass through the bar and then go from end to end through
the atmosphere. Air is a poor conductor of electricity, so that if we can
find a shorter way to conduct the current from the north pole to the
south pole, the efficiency of the magnet is increased.
This is accomplished by means of the well-known horseshoe magnet,
where the two ends (N, S) are brought close together, as in Fig. 8.
THE EARTH AS A MAGNET.--The earth is a huge magnet and the
magnetic lines run from the north pole to the south pole around all
sides of the globe.
[Illustration: Fig. 8. HORSESHOE MAGNET]
The north magnetic pole does not coincide with the true north pole or
the pivotal point of the earth's rotation, but it is sufficiently near for all

practical purposes. Fig. 9 shows the magnetic lines running from the
north to the south pole.
WHY THE COMPASS POINTS NORTH AND SOUTH.--Now, let us
try to ascertain why the compass points north and south.
Let us assume that we have a large magnet (A, Fig. 10), and suspend a
small