Such an instrument is the beautiful “mirror” galvanometer of Lord Kelvin, Ex-President of the Royal Society, which we illustrate in figure 52, where C is a coil of wire with a small magnetic needle suspended in its heart, and D is a steel magnet supported over it.  The needle (M figure 53) is made of watch spring cemented to the back of a tiny mirror the size of a half-dime which is hung by a single fibre of floss silk inside an air cell or chamber with a glass lens G in front, and the coil C surrounds it.  A ray of light from a lamp L (figure 52) falls on the mirror, and is reflected back to a scale S, on which it makes a bright spot.  Now, when the coil C is connected between the end of the cable and the earth, the signal current passing through it causes the tiny magnet to swing from side to side, and the mirror moving with it throws the beam up and down the scale.  The operator sitting by watches the spot of light as it flits and flickers like a fire-fly in the darkness, and spells out the mysterious message.

A condenser joined in the circuit between the cable and the receiver, or between the receiver and the earth, has the effect of sharpening the waves of the current, and consequently of the 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.