[Illustration:  FIG. 11.—­MAGNETO-ELECTRIC INDUCTION.]

EXTRA CURRENT.

An experiment showing the extra or self-induced current consists in arranging the motor as shown in Fig. 6, and connecting wire with each conductor leading from the battery to the motor, as shown in Fig. 13.  If these wires are grasped one in each hand while the motors is in motion, a slight shock will be felt, providing the hands are moistened.

TELEGRAPH SOUNDERS AND KEYS.

The cabinet contains material for two telegraph sounders and keys which will enable the user to establish a short telegraph line with a single cell of battery.  The armature, m, may be lifted from its pivot so as to permit of slipping one of the coils, A, on to the round magnetic core of the sounder.  The armature is then replaced, as shown in Fig. 14, and the small retractile spring at the rear of the instrument is arranged to draw down the shorter arm of the armature lever.  One of the terminals of the coil, A, is connected with the turned up pivoted end of the telegraph key, o, on the same base.  The other terminal is connected with one pole of the battery and the contact point of the key is connected with the other pole of the battery, as shown.  By swinging the key laterally, so as to remove it from the contact point, it will be found that every touch of the key produces a movement of the sounder lever.  To connect the two instruments together upon a line, it is only necessary to connect the two keys with one wire and the terminals of the two coils with another wire, cutting one of these wires and inserting the battery.

[Illustration:  FIG. 15.—­MAGNETIC FIGURES.]

As soon as the operator ceases to work his instrument he should place the key in contact with the contact point, and cause it to remain there by slipping the end of the key under the head of the screw provided for that purpose.  The other operator can then proceed to send his message.

Those who desire to practice telegraphy should learn the Morse telegraphic code.

MAGNETIC FIGURES.

By arranging the coil so as to form an electro-magnet, as before described, and holding the magnet under a plate of glass sprinkled with fine iron filings, as shown in Fig. 15, and then sending a current through the magnet, at the same time jarring the glass by striking it with a lead pencil, a magnetic figure will be formed which is sometimes called the magnetic spectrum.  By connecting the terminals of the coils diagonally with each other, and connecting the remaining terminals with the battery, two like poles will be formed, and the magnetic figures will have an entirely different appearance, owing to the repulsion between the two like polarities.  Different figures may be produced by using the solenoids without the iron cores.