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.

Fig. 29.

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.

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.

40. Terrestrial Magnetism. As the compass-needle points to the north and south, the earth must act like a magnet. There is a place very far north, about a thousand miles from the north pole of the earth, which is called the earth's north magnetic pole. Compass-needles point to this place, and not to the earth's real north pole. You can see, then, that if a compass be taken north of this magnetic pole, its north pole will point south. Lines of force pass from the earth's north magnetic pole through the air on all sides of the earth and enter the earth's south magnetic pole. The compass-needle, in pointing toward the north magnetic pole, merely takes the direction of the earth's lines of force, just as the particles of iron filings arrange themselves in the magnetic figures.

41. Declination. As the magnetic needle does not point exactly to the north, an angle is formed between the true north and south line and the line of the needle. In Fig. 32 the line marked N S is the true north and south line. The angle of variation, or the declination, is the angle A between the line N S and the compass-needle.