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
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.Fig. 7. Reversed Magnets
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.
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 is a huge magnet and the magnetic lines run from the north pole to the south pole around all sides of the globe.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.
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 magnet (B) above it, so that it is within the magnetic field of the large magnet. This may be done by means of a short pin (C), which is located in the middle of the magnet (B), the upper end of this pin having thereon a loop to which a thread (D) is attached. The pin also carries thereon a pointer (E), which is directed toward the north pole of the bar (B).Fig. 9. Earth's Magnetic Lines
You will now take note of the interior magnetic lines (X), and the exterior magnetic lines (Z) of the large magnet (A), and compare the direction of their flow with the similar lines in the small magnet (B).
The small magnet has both its exterior and its interior lines within the exterior lines (Z) of the large magnet (A), so that as the small magnet (B) is capable of swinging around, the N pole of the bar (B) will point toward the S pole of the larger bar (A). The small bar, therefore, is influenced by the exterior magnetic field (Z).Fig. 10. Two Permanent Magnets Fig. 11. Magnets in the Earth's Magnetic Field
Let us now take the outline represented by the earth's surface (Fig. 11), and suspend a magnet (A) at any point, like the needle of a compass, and it will be seen that the needle will arrange itself north and south, within the magnetic field which flows from the north to the south pole