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. [Illustration: FIG. 279.--The moving magnet induces a current in the coil.] [Illustration: 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. [Illustration: FIG. 281.--A magneto.] [Illustration: FIG. 282.--A shuttle armature.] [Illustration: FIG. 283.--The induced current has a field which opposes the motion of the magnet. The heavy line represents the direction of the induced current.]