thumb|Muirhead syphon recorder
The syphon or siphon recorder is an electromechanical device formerly used as a receiver for submarine telegraph cables. It was 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 previous invention, the 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.
Principle of operation
thumb|upright=1.4|Diagram of a siphon recorder mechanism from 1922
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.
The current from the telegraph line is applied to the coil. The pulses of current representing the Morse code "dots" and "dashes" flowing through the coil create a magnetic field which interacts with the magnetic field of the magnet, creating a torque which causes the coil to rotate slightly about its vertical suspension axis. The wire linkages cause the siphon support plate to rotate about its horizontal axis, swinging the siphon tube across the paper tape. This draws a displacement in the ink line on the tape as long as the current is present in the coil. Thus the ink line on the tape forms a graph of the current in the telegraph line, with displacements representing the "dots" and "dashes" of the Morse code. An operator knowing Morse code later translates the line on the tape to characters of the text message, and types them onto a telegram form.
Kelvin's electrostatic syphon
thumb|upright=2.5|Example of transatlantic radiotelegraph message recorded on paper tape by a siphon recorder at RCA's receiving center in New York City in 1920.
[[File:Syphon recorder output.png|thumb|upright=2.0|Due to poor transmission quality of long telegraph lines the paper tape was often hard to read.