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.
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