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FIG. 4. Lines of Force around a Bar Magnet.

The paths of the lines of force can be 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. The filings arrange themselves in curved lines, diverging from one pole of the magnet and meeting again at the opposite end. 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.

The phenomena of magnetism and its laws form a very important branch of the study of electricity, for they play a part in the construction and operation of almost all electrical apparatus.

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In 1831 Michael Faraday, the great physicist, made the valuable discovery that electric currents are induced in a closed circuit by moving a magnet near it or vice versa, by moving the circuit across the field of force, If a coil of insulated wire be connected in circuit with a sufficiently delicate galvanometer (a galvanometer is an instrument for detecting feeble electric currents) and a bar magnet suddenly plunged into the hollow of the coil as shown in the illustration, a momentary current will be indicated as flowing through the galvanometer while the magnet is being moved in the coil. If the magnet is then rapidly pulled out of the coil another momentary current will be observed to flow in the opposite direction from the former.