signals. The double-current key, which reverses the poles of the
battery and allows the signal currents to be of one length, that
is to say, all "dots," is employed to send the message.

Another receiving instrument employed on most of the longer cables
is the siphon recorder of Lord Kelvin, shown in figure 54, which
marks or writes the message on a slip of travelling paper.
Essentially it is the inverse of the mirror instrument, and
consists of a light coil of wire S suspended in the field between
the poles of a strong magnet M. The coil is attached to a fine
siphon (T5) filled with ink, and sometimes kept in vibration by an
induction coil so as to shake the ink in fine drops upon a slip of
moving paper. The coil is connected between the cable and the
earth, and, as the signal current passes through, it swings to one
side or the other, pulling the siphon with it. The ink, therefore,
marks a wavy line on the paper, which is in fact a delineation of
the rise and fall of the signal current and a record of the
message. The dots in this case are represented by the waves above,
and the "dashes" by the waves below the middle line, as may be
seen in the following alphabet, which is a copy of one actually
written by the recorder on a long submarine cable.

Owing to induction, the speed of signalling on long cables is much
slower than on land lines of the same length, and only reaches
from 25 to 45 words a minute on the Atlantic cables, or 30 to 50
words with an automatic sending-key; but this rate is practically
doubled by employing the Muirhead duplex system of sending two
messages, one from each end, at the same time.

The relation of the telegraph to the telephone is analogous to