the needle will increase with the increase of plate surface submitted
to the action of the battery fluid; and if, when the greatest
deflection is reached, the coils or solenoids are introduced into the
circuit, one after the other, it, will be found that each added coil
diminishes the current, as will be shown by the diminished deflection
of the needle.

[Illustration: FIG. 16.--EXPERIMENT SHOWING THE CURRENT.]


MICROPHONE AND TELEPHONE.

Take two small carbon rods, p p, if procurable, if not, use two
ordinary nails, and connect them up in the circuit of the battery; lay
them upon a thin box so that the rods or nails cross each other, as in
Fig. 17; insert the electromagnet in the circuit; move the coils out a
little beyond the ends of the cores, lay a thin iron plate over the
ends of the coils, then jar the box upon which the bars, p p, are
laid, or drop a pin upon it, or scratch it with a piece of paper, and
the sound will be heard by placing the ear against the iron plate
resting upon the coils of the magnet.


ELECTRO METALLURGY.

Dissolve an ounce of sulphate of copper in a half pint of water; add a
few drops of sulphuric acid; connect with the zinc pole of the battery
the object to be coppered. To the wire connected with the carbon
attach a small plate of copper. Hang the object and the copper plate
in the solution a short distance apart. A deposit of copper will be