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 species of
galvanometer as to imitate the motions of the needle. But before it
reached the Red Sea the cable had broken down, and the instrument was
returned dismantled, to be superseded at length by the siphon recorder,
in which the marking point is a fine glass siphon emitting ink, and the
moving body a light coil of wire hung between the poles of a magnet.
The principle of the siphon recorder is exactly the inverse of the
mirror galvanometer. In the latter we have a small magnet suspended in
the centre of a large coil of wire--the wire enclosing the magnet, which
is free to rotate round its own axis. In the former we have a small
coil suspended between the poles of a large magnet--the magnet enclosing
the coil, which is also free to rotate round its own axis. When a
current passes through this coil, so suspended in the highly magnetic
space between the poles of the magnet, the coil itself experiences a
mechanical force, causing it to take up a particular position, which
varies with the nature of the current, and the siphon which is attached
to it faithfully figures its motion on the running paper.
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