[Illustration:  FIG. 7.—­MAGNETIC EXPERIMENT.]

GALVANOMETER.

By placing one of the coils, A, in the block, D, then placing in the cavity in the top of the block the compass, with the line marked N S arranged at right angles to the axis of the coil, a serviceable galvanometer will be formed (Fig. 8).  By turning the galvanometer so that the needle will point north and south without the current passing, with N underneath one end of the needle, and then connecting the poles of the battery with the terminals of this galvanometer, a deflection of the compass needle will be produced, the direction of which depends upon the direction of the current.

EXPERIMENTS SHOWING THE EFFECTS OF RESISTANCE.

By placing the galvanometer in the circuit of the battery, as shown in Fig. 9, and noting the deflection of the needle, it will be ascertained that a certain amount of current is flowing.  Now, by placing in the circuit, in addition to the galvanometer, the remaining coil of the magnet, thus introducing considerable resistance, the current will be diminished, as shown by a smaller deflection of the needle.

RESISTANCE OF A FLUID CHANGED BY THE ADDITION OF ANOTHER FLUID.

A very pretty and instructive experiment may be performed by arranging the apparatus as shown in Fig. 10, with the copper strips, H H, inserted in clean water and the galvanometer placed in the circuit.  The deflection of the galvanometer needle will be very slight, showing that the resistance of clean water is considerable.  A few drops of sulphuric acid or even vinegar will increase the conductivity of the water so as to produce a marked deflection of the galvanometer needle.

Common salt added to the water will produce the same effect.

MAGNETIC ELECTRIC INDUCTION.

By placing one of the coils, A, on the magnet frame, B, and connecting it by the wires with the galvanometer, arranged as before described, and bringing the permanent magnet, F, suddenly against the poles of the magnet, as shown in Fig. 11, a current will be induced in the coil, which, in passing through the galvanometer, causes the needle to be deflected in one direction, and when the permanent magnet is suddenly removed from the electro-magnet, a current will be set up in the opposite direction, which will cause a deflection of the needle of the galvanometer in the opposite direction.

INDUCTION COIL.

By placing both coils, A, upon the bundle of soft iron wires, G, connecting one of them with the terminals of the battery, as shown in Fig. 12, and holding the terminals of the other coil in the moistened thumb and fingers of the two hands, when the battery circuit is opened and closed by touching one of the wires to the battery, and removing it, a slight shock will be felt from the coil which is disconnected from the battery.  By placing a coarse file in the circuit and drawing one of the terminals along the file the circuit will be rapidly interrupted.  This shock is due to the current induced in the detached coil by the magnetism of the bundle of wires.