lagging undulations or waves, which tend to run together or
coalesce. The result is that the separate signal currents which
enter a long cable issue from it at the other end in one
continuous current, with pulsations at every signal, that is to
say, in a lapsing stream, like a jet of water flowing from a
constricted spout. The receiving instrument must be sufficiently
delicate to manifest every pulsation of the current. Its
indicator, in fact, must respond to every rise and fall of the
current, as a float rides on the ripples of a stream.

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