Syphon recorder
Electromechanical receiver that recorded Morse code as ink on paper tape.
The syphon recorder, an electromechanical device, served as a receiver for submarine telegraph cables. It was invented by William Thomson, 1st Baron Kelvin, in 1867. The machine automatically transcribed incoming telegraph messages by drawing a wiggling ink line onto a roll of paper tape. Later, a trained telegrapher would read this tape, converting the pulses representing Morse code dots and dashes into the characters of the text message.
This device replaced Thomson’s earlier invention, the mirror galvanometer, as the standard receiver for submarine cables. The mirror galvanometer required two operators—one to watch and call out the signal, another to write down the characters—whereas the syphon recorder allowed long cables to be operated with only a few volts at the sending end. Its use later expanded to ordinary telegraph lines and radiotelegraphy radio receivers. A key benefit was that no operator needed to constantly monitor the line for incoming messages. The paper tape also preserved a record of the actual signal before translation, enabling checks for translation errors.
The syphon recorder operates on the principle of a d’Arsonval galvanometer. A lightweight coil of wire is suspended between the poles of a permanent magnet, allowing it to turn freely. This coil connects via two wire linkages to a metal plate that supports the siphon, pivoting on a horizontal suspension thread. From this plate, a narrow glass siphon tube hangs vertically, its tip nearly touching a paper tape. Motorized rollers pull the tape at a constant speed beneath the siphon pen. Ink is drawn up from a reservoir into the tube by siphon action and exits through a tiny orifice at the end, drawing a line on the moving tape. To avoid affecting the coil’s motion, the siphon tube never contacts the paper—only the ink does.
Current from the telegraph line flows through the coil. The pulses of current representing Morse code dots and dashes generate a magnetic field that interacts with the permanent magnet’s field, creating a torque that rotates the coil slightly around its vertical suspension axis. The wire linkages then cause the siphon support plate to rotate around its horizontal axis, swinging the siphon tube across the paper tape. This movement displaces the ink line on the tape for as long as current is present in the coil.
- Inventor
- William Thomson, 1st Baron Kelvin
- Year of invention
- 1867
- Predecessor
- Mirror galvanometer
- Application
- Submarine telegraph cables, ordinary telegraph lines, radiotelegraphy radio receivers
Lore & Background
The syphon recorder was invented by William Thomson, 1st Baron Kelvin in 1867, replacing his earlier mirror galvanometer as the standard receiving instrument for submarine telegraph cables. The mirror galvanometer required two operators—one to read the signal and another to write down the characters—whereas the syphon recorder needed no constant monitoring, and its paper tape preserved a record of the message for later translation and error checking. The device allowed long cables to be worked using just a few volts at the sending end.
The recorder operates on the principle of a d'Arsonval galvanometer. A light coil of wire suspended between the poles of a permanent magnet rotates when current pulses from the telegraph line pass through it. The coil is linked to a siphon support plate, which swings a narrow glass siphon tube across a moving paper tape. Ink is drawn up from a reservoir by siphon action and flows through a tiny orifice, drawing a line on the tape without the tube touching the paper. The resulting ink line forms a graph of the current, with displacements representing Morse code dots and dashes.
Later developments included Kelvin's electrostatic syphon, where the ink was electrified to a high potential while the instrument body was grounded, causing the ink to rush through the siphon and discharge as a fine spray onto the paper. Power for the paper tape and the electrostatic machine was usually supplied by Froment's mouse mill motors. Alexander Muirhead developed a simpler vibrating recorder that used a vibrating pen to prevent ink from sticking to the paper.
Reader's Guide
The syphon recorder was significant because it automated the reception of submarine telegraph messages, eliminating the need for two operators at the receiving end. Its paper tape provided a permanent record of the raw signal, allowing translation errors to be checked later. This made long-distance cable telegraphy more practical and reliable, requiring only a few volts at the sending end. The device also spread to ordinary telegraph lines and radiotelegraphy radio receivers, broadening its impact. However, due to poor transmission quality on long lines, the paper tape was often hard to read. The recorder's legacy lies in its role as a key electromechanical interface between electrical signals and human-readable records, bridging the gap between the mirror galvanometer and later automatic printing telegraphs.
Did You Know?
- It replaced Thomson's earlier mirror galvanometer, which required two operators.
- The ink was electrified to a high potential in Kelvin's electrostatic syphon design.
- Alexander Muirhead developed a vibrating recorder that used a pen vibrated by an electromagnet mechanism similar to an electric bell.
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