CXVII
§ 297. It is produced in the following way. Round a
hollow cylinder a silk-covered copper wire is coiled (called the
secondary coil). Into the hollow of this cylinder fits a similar
one (called the primary coal), which consists of less turnings of
a stouter copper wire. The ends of the primary coil are connect
ed with the two poles of a battery, whilst the two ends of the se
condary coil are either connected with a galvanometer or, if
used for psychological purposes, with the human body.
As the effects of this kind of electricity last only for a mo
ment, for utilizing them an apparatus is necessary, by which as
quick as possible the galvanic current may be interrupted and
thus a continual current can be produced (a cog-wheel). Induc
ed currents were discovered by Faraday (1831).
§ 298. Induction by Magnets. If instead of the inward
cylinder (primary coil) a magnetic rod is inserted and the ends
of the two wires (of the secondary coil) are connected with a
Multiplier, the existence of an electric current can be proved.
Instead of the magnetic rod even a rod of soft iron may be
taken and by approaching and removing a magnet may be mag
netised. This proves, that not only by electricity is magnetism
caused, but also that magnetism will produce electricity. Even
the magnetism of the earth may be used for causing induced
currents (Faraday).
§ 299. 9). The chief application of these electromagnets
we see in the Electric Telegraphs. The chief parts of Morse's
telegraph are: 1) The voltaic battery as the source of power
with the line wire, 447, I. The wires may be aerial, subter
ranean and submarine. The aerial wire is that in general use;
it is sustained by wooden posts and insulated by supports of
porcelain. The sub-marine wires are also called Cables. In
the telegraph a wire connects the positive pole with the electro
