=36. Induced Magnetism.= A piece of soft iron may be induced to become a magnet by holding it near a magnet, absolute contact not being necessary. When the soft iron is removed, again, from the influence of the magnet, its magnetism nearly all disappears. It is said to have _temporary_ magnetism; it had _induced_ magnetism. If a piece of soft iron be held near the north pole of a magnet, as in Fig. 27, poles will be produced in the soft iron, the one nearest the magnet being the south pole, and the other the north pole. [Illustration: Fig. 27.] [Illustration: Fig. 28.] =37. Magnetic Field.= If a bar magnet be laid upon the table, and a compass be moved about it, the compass-needle will be attracted by the magnet, and it will point in a different direction for every position given to the compass. This strange power, called magnetism, reaches out on all sides of a magnet. The magnet may be said to act by induction upon the compass-needle. The space around the magnet, in which this inductive action takes place, is called the _magnetic field_. Fig. 28 shows some of the positions taken by a compass-needle when moved about on one side of a bar magnet. [Illustration: Fig. 29.] [Illustration: Fig. 30.] =38. Magnetic Figures= can be made by sprinkling iron filings upon a sheet of paper under which is placed a magnet. Fig. 29 shows a magnetic figure made with an ordinary bar magnet. The magnet was placed upon the table and over this was laid a piece of smooth paper. Fine iron filings were sifted upon the paper, which was gently tapped so that the filings could arrange themselves. As each particle of iron became a little magnet, by induction, its poles were attracted and repelled by the magnet; and when the paper was tapped they swung around to their final positions. Notice that the filings have arranged themselves in lines. These lines show the positions of some of the _lines of magnetic force_ which surrounded the magnet. These lines of force pass from the north pole of a magnet through the air on all sides to its south pole. [Illustration: Fig. 31.] Fig. 30 shows a magnetic figure made from two bar magnets placed side by side, their unlike poles being next to each other. Fig. 31 shows the magnetic figure of a horseshoe magnet with round poles, the poles being uppermost. =39. The Use of Armatures.= A magnet attracts iron most strongly at its poles, because it is at the poles that the greatest number of lines of force pass into the air. Lines of force pass easily through soft iron, which is said to be a good conductor of them. Air is not a good conductor of the lines of force; in order, then, for the lines of force to pass from the north pole of a magnet to its south pole, they must overcome this resistance of the air, unless the armature is in place. A magnet will gradually grow weaker when its armature is left off.