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. IX BATTERIES So far we have only described one way of producing electricity--namely, by means of a dynamo-machine driven by steam or water power. The supply of electricity so obtained is regular and constant as long as the steam or water power is applied to the dynamo. There is another and very different way of producing electricity, and this is by means of a chemical process in what is called a battery.