of the mirror
can be magnified on the screen by the reflected beam of light,
which acts as a long impalpable hand or pointer, render the mirror
galvanometer marvellously sensitive to the current, especially when
compared with other forms of receiving instruments. Messages have been
sent from England to America through one Atlantic cable and back
again to England through another, and there received on the mirror
galvanometer, the electric current used being that from a toy battery
made out of a lady's silver thimble, a grain of zinc, and a drop of
acidulated water.

The practical advantage of this extreme delicacy is, that the signal
waves of the current may follow each other so closely as almost entirely
to coalesce, leaving only a very slight rise and fall of their crests,
like ripples on the surface of a flowing stream, and yet the light spot
will respond to each. The main flow of the current will of course shift
the zero of the spot, but over and above this change of place the spot
will follow the momentary fluctuations of the current which form the
individual signals of the message. What with this shifting of the zero
and the very slight rise and fall in the current produced by rapid
signalling, the ordinary land line instruments are quite unserviceable
for work upon long cables.

The mirror instrument has this drawback, however--it does not 'record'
the message. There is a great practical advantage in a receiving
instrument which records its messages; errors are avoided and time
saved. It was to supply such a desideratum for cable work that Sir
William Thomson invented the siphon recorder, his second important
contribution to the province of practical telegraphy. He aimed at giving
a GRAPHIC representation of the varying strength of the current, just as
the mirror galvanometer gives a visual one. The difficulty of producing
such a recorder was, as he himself says, due to a difficulty in
obtaining marks from a very light body in rapid motion, without
impeding that motion. The moving body must be quite free to follow the
undulations of the current, and at the same time must record its motions
by some indelible mark. As early as 1859, Sir William sent out to
the Red Sea cable a piece of apparatus with this intent. The marker
consisted of a light platinum wire, constantly emitting sparks from a
Rhumkorff coil, so as to perforate a line on a strip of moving
paper; and it was so connected to the movable needle of a sp