Telecommunications collection : Catalogue ” Electrical Telegraphy : Canstatt single-needle telegraph (rwth-aachen.de)
In 1820, Paul Schilling von Canstatt (1786-1837) built a needle telegraph consisting of a Schweigger galvanometer (or multiplier), above which an astatic needle (i.e. the needles do not align with the earth’s magnetic field) was attached in a suspension device. The needles do not align themselves with the earth’s magnetic field) and on the thread just above the needle hung a drawing disc on which letters, numbers or simply agreed symbols – such as the plus and minus sign or black and white – were displayed so that one could clearly see when current was flowing through the wire. This was because the needles aligned themselves either to the left or to the right, depending on the direction of the current, and so did the disc, which then showed either the black or the white side. When at rest, the narrow side of the disc was visible.
The black and white disc was used for a series code: “A” was black, white; “B” black, black, black; “C” black, white, white, etc.
Because of the deflection, black was equated with r(right) and white with l(left).