iece of zinc and copper joined by a wire be dipped in a
liquid--generally weak sulphuric acid--which will act chemically on
the metals, a current is produced. Such an arrangement is called a
couple, or cell. If many cells are connected, then it is called a
battery.

What is Thermo-electricity?

If two bars of any unlike metal--for example, antimony and bismuth--be
soldered together at one end, and the other ends be connected by a
wire and then the soldered end heated, a current will flow.

What effects are produced by currents?

They produce heat, light, decomposition and combination in liquid
chemical compounds; they melt all metals, excite magnetism, and in the
animal body excite movements of the muscles.

Can you specify these effects?

A strong battery produces heat in such a degree that all metals can be
melted. Light is produced in flashes, or if the end of the leading
wires are connected with two pencils of hard carbon, and brought very
near together, then a brilliant light, or arc, called the voltaic arc,
is produced. This is the dazzling bright light which we call electric
light. The chemical effect of a current in decomposing compound
substances is called electrolysis. In this way water can be decomposed
into its compounds, hydrogen and oxygen; copper sulphate into sulphur
and metallic copper, etc. In this way we can deposit strong adherent
films of metal on the surface of any conductor; for if the article to
be coated be attached to the negative electrode of a battery, and
dipped into a solution of the metal with which we desire to coat the
article, say copper or silver, and the positive electrode be attached
to a plate of copper and also dipped into a liquid, when the current
passes, the metal will be decomposed and deposited in a uniform layer
over the article at the negative electrode. This process is called
_electro-plating_.

What is Electrotyping?

It is the process of copying medals, type, wood-cuts, engraved copper
and steel plates, etc., by means of electrical deposition. It is
chiefly used for making, from the ordinary movable types, plates of
fixed metallic types, for printing books.

Describe the process.

The article to be copied is first covered with black-lead, and then a
mould is made of it in wax or gutta-percha. This mould is placed in a
solution of sulphate of copper, and attached to the negative pole of
the battery, while a plate of copper is hung from the positive pol