traversed, as we know, by an electric current, which makes the
round of the circuit, and only flows when that circuit is
complete. However long the wire may be, however far it may run
between the poles, the current will follow all its windings, and
finish its course from pole to pole of the battery. You may lead
the wire across the ocean and back, or round the world if you
will, and the current will travel through it.

The moment you break the wire or circuit, however, the current
will stop. By its electromotive force it can overcome the
resistance of the many miles of conductor; but unless it be
unusually strong it cannot leap across even a minute gap of air,
which is one of the best insulators.

If, then, we have a simple device easily manipulated by which we
can interrupt the circuit of the battery, in accordance with a
given code, we shall be able to send a series of currents through
the wire and make sensible signals wherever we choose. These signs
can be produced by the deviation of a magnetic needle, as Laplace
pointed out, or by causing an electro-magnet to attract soft iron,
or by chemical decomposition, or any other sensible effect of the
current.

Ampere developed the idea of Laplace into a definite plan, and in
1830 or thereabout Ritchie, in London, and Baron Schilling, in St.
Petersburg, exhibited experimental models. In 1833 and afterwards
Professors Gauss and Weber installed a private telegraph between
the observatory and the physical cabinet of the University of
Gottingen. Moreover, in 1836 William Fothergill Cooke, a retired
surgeon of the Madras army, attending lectures on anatomy at the