*Magnetic Circuit.* The paths of the lines of force can be easily demonstrated by placing a piece of paper over a bar magnet and then sprinkling iron filings over the paper, which should be jarred slightly in order that the filings may be drawn into the magnetic paths. [Illustration: Fig. 13.—A Magnetic "Phantom," Showing the Field of Force about a Magnet.] The filings will arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite pole. The lines of force are considered as extending outward from the north pole of the magnet, curving around through the air to the south pole and completing the circuit back through the magnet. [Illustration: Fig. 14.—Magnetic Phantom showing the Lines of Force about a Horseshoe Magnet.] Figure 14 shows the lines of force about a horseshoe magnet. It will be noticed that the lines cross directly between the north and south poles. The difference between the magnetic fields produced by like and unlike poles is shown in Figure 15. [Illustration: Fig. 15.—Lines of Force between Like and Unlike Poles.] A study of this illustration will greatly assist the mind in conceiving how attraction and repulsion of magnetic poles take place. It will be noticed the lines of force between two north poles resist each other and meet abruptly at the center. The lines between a north and a south pole pass in regular curves. *The Earth is a Great Magnet.* The direction assumed by a compass needle is called the _magnetic meridian_. The action of the earth on a compass needle is exactly the same as that of a permanent magnet. The fact that a magnetized needle places itself in the magnetic meridian is because the earth is a great magnet with lines of force passing in a north and south direction. The compass needle does not generally point exactly toward the true North. This is because the magnetic pole of the earth toward which the needle points is not situated at the same place as the geographical pole. *Magnetic Dip.* If a sewing-needle is balanced so as to be perfectly horizontal when suspended from a silk thread and is then magnetized, it will be found that it has lost its balance and that the _north_ end points slightly downward. [Illustration: Fig. 16.—A Simple Dipping Needle.] This is due to the fact that the earth is round and that the magnetic pole which is situated in the far North is therefore not on a horizontal line with the compass, but below such a line. A magnetic needle mounted so as to move freely in a vertical plane, and provided with a scale for measuring the inclination, is called a *Dipping Needle.* A dipping needle may be easily made by thrusting a knitting-needle through a cork before it has been magnetized. A second needle is thrust through at right angles to the first and the arrangement carefully balanced, so that it will remain horizontal when resting on the edge of two glasses.