Bain also proposed to make the
coil record the message by printing it in type; and he developed the
idea in a subsequent patent.
Next year, on December 31st, 1844, he projected a mode of measuring the
speed of ships by vanes revolving in the water and indicating their
speed on deck by means of the current. In the same specification he
described a way of sounding the sea by an electric circuit of wires, and
of giving an alarm when the temperature of a ship's hold reached a
certain degree. The last device is the well-known fire-alarm in which
the mercury of a thermometer completes an electric circuit, when it
rises to a particular point of the tube, and thus actuates an electric
bell or other alarm.
On December 12, 1846, Bain, who was staying in Edinburgh at that time,
patented his greatest invention, the chemical telegraph, which bears his
name. He recognised that the Morse and other telegraph instruments in
use were comparatively slow in speed, owing to the mechanical inertia of
the parts; and he saw that if the signal currents were made to pass
through a band of travelling paper soaked in a solution which would
decompose under their action, and leave a legible mark, a very high
speed could be obtained. The chemical he employed to saturate the paper
was a solution of nitrate of ammonia and prussiate of potash, which left
a blue stain on being decomposed by the current from an iron contact or
stylus.
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