Although a motor is made like a dynamo, we will show a different form
of machine from the dynamo already illustrated, because it will help
you to understand more easily. (Fig. 28.)
Here we have an electromagnet with its poles, and an iron armature
wound with wire, just as in the dynamo we have described, except that
its form is different.
[Illustration: Fig. 28]
A commutator and brushes are also used, but the electricity, instead
of being taken away from the brushes, is taken _to_ them by the wires
connected with them. Two wires are also connected which take part of
the electricity around the magnet, just as in the dynamo.
Now, when the volts pressure and ampères of electricity coming from a
dynamo or battery are turned into the wires leading to the brushes of
the motor, they go through the commutator into the armature and round
the magnet, and so create the lines of force at the poles and magnetize
the iron of the armature.
Let us see what the effect of this is.
The poles of the magnet become north and south, and the four ends on
the armature also become north and south, two of each.
By referring to Fig. 28 again we shall see what takes place.
The north pole of the magnet is doing two things: it is repelling, or
forcing away, the upper north pole of the armature and at the same time
drawing toward itself the lower south pole of the armature.
In the mean time the south pole of the magnet is repelling the south
pole of the armature and at the same time drawing toward itself the
north pole of the armature.
This, of course, makes the armature turn around, and the same poles
are again presented to the magnet, when they are acted upon in the
same manner, which makes the armature revolve again, and this action
continues as long as electricity is brought through the wires to the
brushes. Thus, the armature turns around with great speed and strength,
and will then drive a machine to which it is attached.
The speed and strength of the motor are regulated by the amount of iron
and wire upon it, and by the volts pressure and ampères of electricity
supplied to the brushes. Motors are made from a small size that will
run a sewing-machine up to a size large enough to run a railway train,
and are often operated through wires at a great distance from the place
where the electricity is being made, sometimes miles away.
They are also made in a great many different forms, but the principle
is practically the same as we have just described to you.