Chemical Recorder.
A form of telegraphic recorder in which the characters, often of the
Morse alphabet or some similar one, are inscribed on chemically prepared
paper by decomposition affecting the compound with which the paper is
charged. In the original chemical recorder of Bain, the instrument was
somewhat similar to the Morse recorder, except that the motionless
stylus, S, always pressing against the paper was incapable of making any
mark, but being of iron, and the paper strip being impregnated with
potassium ferrocyanide, on the passage of a current a stain of Prussian
blue was produced where the stylus touched the paper. The current passes
from the line by way of the iron stylus, through the paper, and by way
of a brass surface, M, against which the paper is held and is pressed by
the stylus, to the earth. This recorder is extremely simple and has no
part to be moved by the current. The solution in which the paper is
dipped contains a mixture of potassium ferrocyanide and ammonium
nitrate. The object of the latter is to keep the paper moist. In recent
recorders a solution of potassium iodide has been used, which gives a
brown stain of free iodine, when the current passes. This stain
disappears in a few days.
Fig. 83. BAIN'S TELEGRAPH EMPLOYING CHEMICAL RECORDER.
In the cut, R is the roll of paper, B is a tank of solution with roll,
W1, for moistening the paper; M is the brass surface against which the
stylus, S, presses the paper, P P; W, W are feed rollers; T is the
transmitting key, and zk the battery; Pl, Pl are earth plates. The
apparatus is shown duplicated for each end.