Electricity for Boys | Page 7

unseen electric current into motion which may be observed by the human eye. Without it electricity would be a useless agent to man.
While the electro-magnet is, therefore, the form of device which is almost wholly used, it is necessary, first, to understand the principles of the permanent magnet.
MAGNETISM.--The curious force exerted by a magnet is called magnetism, but its origin has never been explained. We know its manifestations only, and laws have been formulated to explain its various phases; how to make it more or less intense; how to make its pull more effective; the shape and form of the magnet and the material most useful in its construction.
[Illustration: Fig 5. PLAIN MAGNET BAR]
MATERIALS FOR MAGNETS.--Iron and steel are the best materials for magnets. Some metals are non-magnetic, this applying to iron if combined with manganese. Others, like sulphur, zinc, bismuth, antimony, gold, silver and copper, not only are non-magnetic, but they are actually repelled by magnetism. They are called the diamagnetics.
NON-MAGNETIC MATERIALS.--Any non-magnetic body in the path of a magnetic force does not screen or diminish its action, whereas a magnetic substance will.
In Fig. 5 we show the simplest form of magnet, merely a bar of steel (A) with the magnetic lines of force passing from end to end. It will be understood that these lines extend out on all sides, and not only along two sides, as shown in the drawing. The object is to explain clearly how the lines run.
[Illustration: Fig. 6. SEVERED MAGNET]
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.
[Illustration: Fig. 7. REVERSED MAGNETS]
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 horseshoe magnet, where the two ends (N, S) are brought close together, as in Fig. 8.
THE EARTH AS A MAGNET.--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.
[Illustration: 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.
WHY THE COMPASS POINTS NORTH AND SOUTH.--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).
[Illustration: 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