The magnets made of iron are called electromagnets because they exhibit magnetism only when the ampères of current of electricity are flowing around them. They also have two poles, north and south, as have permanent magnets. Electromagnets are used in nearly all electrical instruments, not only because they are stronger than permanent magnets, but because they can be made to act instantly by passing a current of electricity through them at the most convenient moment, as you will see when we explain some of the electrical instruments which are used to produce certain effects. (Fig. 4.) [Illustration: Fig. 4] Of course there are a great many different shapes in which magnets are made. The simplest is the _bar magnet_, which is simply a flat or round piece of iron or steel. Suppose you made a magnet of a flat piece of steel and put on top of it a sheet of paper, and then threw on the paper some iron filings, you would see them arrange themselves as is shown in the following sketch. (Fig. 5.) The filings would always arrange themselves in this shape, no matter how large or small the magnets were. And, if you were to cut it into two or half a dozen pieces, each piece would have the same effect. This shows you that each piece would itself become a magnet and would have its poles exactly as the large one had. [Illustration: Fig. 5] Now, we have another curious thing to tell you about magnets. If you present the north pole of a magnet to the south pole of another magnet, they will attract and hold fast to each other, but if you present a south pole to another south pole, or a north pole to a north pole, they will repel each other, and there will be no attraction. You can perform some interesting experiments by reason of this fact. We will give you one of them. Take, say, a dozen needles and draw them several times in the same direction across the ends of a magnet so that they become magnetized. Now stick each needle half-way through a piece of cork, and put the corks, with the needles sticking through them, into a bowl of water. Then take a bar magnet and bring it gradually toward the middle of the bowl and you will see the corks advance or back away from the magnet. If the ends of the needles sticking up out of the water are south poles and the end of the magnet you present is a north pole, the needles will come to the center; but will go to the side of the bowl if you present the south pole. You can vary this pretty experiment by turning up the other ends of part of the needles.