296. Current Induced by a Magnet.—The discovery in 1819 that a current in a conductor can deflect a magnetic needle or that it has a magnetic effect, led to many attempts to produce an electric current by means of a magnet. It was not until about 1831, however, that Joseph Henry in America and Michael Faraday in England, independently discovered how to accomplish this important result.
At the present time, voltaic cells produce but a very small part of the current electricity used. Practically all that is employed for power, light, heat, and electrolysis is produced by the use of magnetic fields, or by electromagnetic induction.
297. Laws of Induced Currents.[M]—To illustrate how a current can be produced by electromagnetic induction:
Connect a coil of 400 or more turns of No. 22 insulated copper wire to a sensitive galvanometer. (See Fig. 279.) Now insert a bar magnet in the coil. A sudden movement of the galvanometer will be noticed, indicating the production of a current. When the magnet stops moving, however, the current stops, and the coil of the galvanometer returns to its first position. If now the magnet is removed, a movement of the galvanometer coil in the opposite direction is noticed. This action may be repeated as often as desired with similar results.
Careful experiments have shown that it is the magnetic field of the magnet that produces the action, and that only when the number of lines of force in the coil is changing do we find a current produced in the coil. These facts lead to Law I. Any change in the number of magnetic lines of force passing through or cut by a coil will produce an electromotive force in the coil. In the account of the experiment just given, electric currents are produced, while in Law I, electromotive forces are mentioned. This difference is due to the fact that an E.M.F. is always produced in a coil when the magnetic field within it is changed, while a current is found only when the coil is part of a closed circuit. The inductive action of the earth's magnetic field (see Fig. 280), may be shown by means of a coil of 400 to 500 turns a foot in diameter.
Fig. 279.—The moving magnet induces a current in the coil.
Fig. 280.—A current may be induced by turning the coil in the earth's magnetic field.
Connect its ends to a sensitive galvanometer and hold it at right angles to the earth's field. Then quickly revolve the coil through 180 degrees and note the movement of the galvanometer. Reverse the coil and the galvanometer swings in the opposite direction.
If the magnet in Fig. 279 is moved in and out of the coil at first slowly and later swiftly, small and large deflections of the galvanometer coil are noticed. The quicker the movement of the magnetic field the greater are the galvanometer deflections produced. This leads to Law II. The electromotive forces produced are proportional to the number of lines of force cut per second.
298. The magneto is a device that illustrates the laws of induced currents stated in Art. 297. The magneto (see Fig. 281), consists of several permanent, "U"-shaped magnets placed side by side. Between the poles of these magnets is placed a slotted iron cylinder having a coil of many turns of fine insulated copper wire wound in the slot as in Fig. 282. The cylinder and coil form what is called an armature. The armature is mounted so as to be revolved between the poles of the "U"-shaped magnets by means of a handle. As the armature revolves, the lines of force from the magnets pass through the coil first in one direction and then in the other. This repeated change in the lines of force passing through the coil produces an E.M.F. which may be felt by holding in the hands the two wires leading from the armature coil. On turning the armature faster the current is felt much stronger, showing that the E.M.F. in the coil increases as the rate of cutting the magnetic lines of force by the coils increases.