60.--Condenser]

In many ways a deep-sea cable exaggerates in an instructive manner the
phenomena of telegraphy over long aerial lines. The two ends of a cable
may be in regions of widely diverse electrical potential, or pressure,
just as the readings of the barometer at these two places may differ
much. If a copper wire were allowed to offer itself as a gateless
conductor it would equalize these variations of potential with serious
injury to itself. Accordingly the rule is adopted of working the cable
not directly, as if it were a land line, but indirectly through
condensers. As the throb sent through such apparatus is but momentary,
the cable is in no risk from the strong currents which would course
through it if it were permitted to be an open channel.

[Illustration: Fig. 61.--Reflecting galvanometer L, lamp; N, moving spot
of light reflected from mirror]

A serious error in working the first cables was in supposing that they
required strong currents as in land lines of considerable length. The
very reverse is the fact. Mr. Charles Bright, in _Submarine Telegraphs_,
says:

"Mr. Latimer Clark had the conductor of the 1865 and 1866 lines joined
together at the Newfoundland end, thus forming an unbroken length of
3,700 miles in circuit. He then placed some sulphuric acid in a very
small silver thimble, with a fragment of zinc weighing a grain or two.
By this primitive agency he succeeded in conveying signals through twice
the breadth of the Atlantic Ocean in little more than a second of time
after making contact. The deflections were not of a dubious character,
but full and strong, from which it was manifest than an even smaller
battery would suffice to produce somewhat similar effects."

[Illustration: Fig. 62.--Siphon recorder]

At first in operating the Atlantic cable a mirror galvanometer was
employed as a receiver. The principle of this receiver has often been
illustrated by a mischievous boy as, with a slight and almost
imperceptible motion of his hand, he has used a bit of looking-glass to
dart a ray of reflected sunlight across a wide street or a large room.
On the same plan, the extremely minute motion of a galvanometer, as it
receives the successive pulsations of a message, is magnified by a
weightless lever of light so that the words are easily read by an
operator (Fig. 61). This beautiful invention comes from the hands of Sir
William Thomson [now Lord Kelvin], who, more than any other electrician,
ha