In overland lines the current traverses the wire suddenly, like a
bullet, and at its full strength, so that if the current be sufficiently
strong these instruments will be worked at once, and no time will be
lost. But it is quite different on submarine cables. There the current
is slow and varying. It travels along the copper wire in the form of a
wave or undulation, and is received feebly at first, then gradually
rising to its maximum strength, and finally dying away again as slowly
as it rose. In the French Atlantic cable no current can be detected by
the most delicate galvanoscope at America for the first tenth of a
second after it has been put on at Brest; and it takes about half a
second for the received current to reach its maximum value. This is
owing to the phenomenon of induction, very important in submarine
cables, but almost entirely absent in land lines. In submarine cables,
as is well known, the copper wire which conveys the current is insulated
from the sea-water by an envelope, usually of gutta-percha. Now the
electricity sent into this wire INDUCES electricity of an opposite kind
to itself in the sea-water outside, and the attraction set up between
these two kinds 'holds back' the current in the wire, and retards its
passage to the receiving station.
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