Paul Schilling von Canstatt (1786-1837) built a needle telegraph in 1820, which consisted of a Schweiggers’ galvanometer (or multiplier), above which in a suspension device an astatic needle (ie. 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 either to the left or to the right depending on the direction of the current, and so did the disk, which then showed either the black or the white side. In the idle state, the narrow side of the disk could be seen.

The black and white disc was used for a series code: “A” was black, white; “B” black, black, black; “C” black, white, white, and so on.

Because of the deflection, black was equated with r(right) and white with l(left).

Note

The code table at Kryptografie.de is incorrect. The correct table can be found here:

• Volker Aschoff, Paul Schilling von Canstatt und die Geschichte des elektromagnetischen Telegraphen. ISBN 3-486-20691-5
• T.P.Shaffner, The Telegraph Manual, 1867;
  The telegraph manual : a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern : with six hundred and twenty-five illustrations : Shaffner, Tal. P. (Taliaferro Preston), 1818-1881 : Free Download, Borrow, and Streaming : Internet Archive