Siphon recorder

The syphon or siphon recorder is an obsolete electromechanical device used as a receiver for submarine telegraph cables invented by William Thomson, 1st Baron Kelvin in 1867. It automatically records an incoming telegraph message as a wiggling ink line on a roll of paper tape. Later a trained telegrapher would read the tape, translating the pulses representing the "dots" and "dashes" of the Morse code to characters of the text message.

The syphon recorder replaced Thomson’s mirror galvanometer as the standard receiving instrument for submarine telegraph cables, allowing long cables to be worked using just a few volts at the sending end. The disadvantage of the mirror galvanometer was that it required two operators, one with a steady eye to read and call off the signal, the other to write down the characters received. Its use spread to ordinary telegraph lines and radiotelegraphy radio receivers. A major advantage of the syphon recorder was that no operator has to monitor the line constantly waiting for messages to come in. The paper tape preserved a record of the actual message before translation to text, so errors in translation could be checked.

The siphon recorder works on the principle of a d'Arsonval galvanometer. A light coil of wire is suspended between the poles of a permanent magnet so it can turn freely. The coil is attached via two wire linkages to the metal plate siphon support, which pivots on a horizontal suspension thread. From this plate a narrow glass siphon tube hangs down vertically with its end almost touching a paper tape. The paper tape is pulled by motorized rollers at a constant speed under the siphon pen. Ink is drawn up from a reservoir into the tube by siphon action and comes out a tiny orifice in the end of the siphon tube, drawing a line down the moving paper tape. In order not to affect the motion of the coil, the siphon tube itself never touches the paper, only the ink.

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