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.
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[p. 22] horseshoe magnet, where the two ends (N, S) are brought close together, as in Fig. [8].