From this one may be led to believe that the more turns of wire an electromagnet possesses, the stronger it will be, and while to a certain extent this is true, it should be remembered that it is not simply turns that count but ampere turns and if the number of turns of wire is increased beyond a certain point the resistance of the coil to the electric current will become so great that the current in amperes flowing through the coil is greatly reduced and consequently also the magnetism is decreased.
FIG. 5.—An increase in the number of turns of wire has resulted in considerable increase in the magnetism and this magnet is able to lift a much greater weight than that shown in Figure 4.
It will be found that the magnetism of an electromagnet is strongest at the ends. These places are called the poles.
If you bring one pole of a small electromagnet, formed by winding a nail with a few turns of wire, near a compass needle, you will find that it will attract one end of the compass needle and repel the other. The end of the compass needle which points North is called a North pole. The ends of the electromagnet which attracts the North pole of the compass needle is a South pole.
One of the most important laws of magnetism is that like poles repel each other and unlike poles attract each other. A North and a South pole therefore tend to pull toward each other, whereas two North poles or two South poles repel one another.
Figure 6 illustrates the principle of an electric motor.
It consists of a bar of iron marked "A" called the Armature and wound with a coil of wire called the armature winding. The armature is the part of the motor which revolves.
FIG. 6.—The Principle of the Electric Motor.