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.
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The principle of the telegraph siphon recorder is exactly the inverse of the mirror galvanometer. In the latter we have a small magnet suspended in the centre of a large coil of wire—the wire enclosing the magnet, which is free to rotate round its own axis. In the former we have a small coil suspended between the poles of a large magnet—the magnet enclosing the coil, which is also free to rotate round its own axis. When a current passes through this coil, so suspended in the highly magnetic space between the poles of the magnet, the coil itself experiences a mechanical force, causing it to take up a particular position, which varies with the nature of the current, and the siphon which is attached to it faithfully figures its motion on the running paper. The point of the siphon does not touch the paper, to avoid impeding the motion of the coil.[1]
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.
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.[1] The recording pen was suspended on a thin wire, vibrated by an electromagnet and break contact, similar to that of an electric bell.