The syphon or siphon recorder is an item of telecommunications equipment invented by William Thomson, 1st Baron Kelvin in 1858. It was used to automatically record the receipt of a telegraph message, as a wiggling ink line on a roll of paper tape. Little skill was required to record the message, but a trained telegrapher was still required to read and understand it.
In many ways, it anticipated the modern inkjet printer though it seldom operated reliably.
The principle behind the siphon recorder is the inverse of the mirror galvanometer. In the mirror galvanometer, a small magnet, which is free to rotate round its own axis, is suspended in the centre of a large coil of wire. In the siphon recorder, a small coil is suspended between the poles of a large magnet. When a current passes through the coil, the interaction between the electric and magnetic fields produces a mechanical force on it dependent on the electric current. The ink siphon is attached to the coil, and therefore the position at which ink is distributed on the paper varies accordingly. In order not to affect the motion of the coil, the siphon itself never touches the paper.
Kelvin's electrostatic syphon
The siphon and an ink reservoir are together supported by an ebonite bracket, separate from the rest of the instrument, and insulated from it. This separation permits the ink to be electrified to a high potential while the body of the instrument, including the paper and metal writing tablet, are grounded, and at low potential. The tendency of a charged body is to move from a place of higher to a place of lower potential, and consequently the ink tends to flow downwards to the writing tablet. The only avenue of escape for it is by the fine glass siphon, and through this it rushes accordingly and discharges itself upon the paper. The natural repulsion between its like-electrified particles causes the shower to issue in spray. As the paper moves over the pulleys a delicate hair line is marked, straight when the syphon is stationary, but curved when the siphon is pulled from side to side by the oscillations of the signal coil.
Power to pull the roll of paper tape through the syphon recorder was usually supplied by one Froment's mouse mill motors. These also drove an electrostatic machine to generate the electricity to power the syphon.
Muirhead's vibrating recorder
A simpler mechanism was developed by Alexander Muirhead. This used a vibrating pen to avoid the same problem of the ink sticking to the paper. The recording pen was suspended on a thin wire, vibrated by an electromagnet and break contact, similar to that of an electric bell.
- Byrn, Edward (1900). The Progress of Invention in the Nineteenth Century (1900 ed.). New York: Munn & Co. pp. 36–37.
- Kennedy, Rankin (1903). Syphon Recorders. Electrical Installations. Vol. V (1903 five-volume ed.). London: Caxton. pp. 79–81.