f the cable
damps the vibration in the conductor. In the Atlantic cable, strong
currents of electricity are poured periodically into one end, and though
much enfeebled when they reach the other they are sufficiently strong to
work a very delicate "mirror galvanometer" (invented by Lord Kelvin),
which moves a reflected ray up and down a screen, the direction of the
movements indicating a dot or a dash. Reversible currents are used in
transmarine telegraphy. The galvanometer is affected like the coils and
small magnet in Wheatstone's needle instrument (p. 128).

Telephonic currents are too feeble to penetrate many miles of cable.
There is telephonic communication between England and France, and
England and Ireland. But transatlantic telephony is still a thing of the
future. It is hoped, however, that by inserting induction coils at
intervals along the cables the currents may be "stepped up" from point
to point, and so get across. Turning to Fig. 64, we may suppose S to be
on shore at the English end, and S_2 to be the _primary_ winding of an
induction coil a hundred miles away in the sea, which magnifies the
enfeebled vibrations for a journey to S_3, where they are again
revived; and so on, till the New World is reached. The difficulty is to
devise induction coils of great power though of small size. Yet science
advances nowadays so fast that we may live to hear words spoken at the
Antipodes.

[16] In 1896 the late Li Hung Chang sent a cablegram from China to
England (12,608 miles), and received a reply, in _seven minutes_.

Chapter IX.

DYNAMOS AND ELECTRIC MOTORS.

A simple dynamo--Continuous-current dynamos--Multipolar
dynamos--Exciting the field magnets--Alternating current
dynamos--The transmission of power--The electric motor--Electric
lighting--The incandescent lamp--Arc lamps--"Series" and "parallel"
arrangement of lamps--Current for electric lamps--Electroplating.

In previous chapters we have incidentally referred to the conversion of
mechanical work into electrical energy. In this we shall examine how it
is done--how the silently spinning dynamo develops power, and why the
motor spins when current is passed through it.

We must begin by returning to our first electrical diagram (Fig. 50),
and calling to mind the invisible "lines of force" which permeate the
ether in the immediate neighbourhood of a magnet's poles, called the
_magnetic field_ of the magnet.

Many year