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Page 42
[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.
[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.
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