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The Siphon Recorder

The cost of the Atlantic Cable made for a very strong incentive to come up with technologies to speed up the operation of the system, in other words to get message onto the cable and off the other end as fast as possible. The Siphon Recorder, initially developed by Sir William Thompson (later Lord Kelvin) in 1867, was one of the earliest such pieces of equipment.

At first the very weak signal at the other end of an Atlantic Cable was detected using a mirror galvanometer - the current passed through a coil on a delicate suspension moving an attached mirror. A beam of light focussed onto the mirror was thus deflected enough to be readily observed. (See a picture of this arrangement on Bill McLaughlin's website).

This necessitated very careful attention at the receiver not to miss any characters in the Morse Code. Clearly a system that actually recorded the signal would be better. The Siphon Recorder achieved this with again a delicate suspension of an arm moved by a coil and a fine glass tube pen that worked by siphoning ink along its hollow stem and which brushed lightly on a moving paper tape. Improved responsiveness was obtained by vibrating the pen slightly to ease its glide over the paper.

This was a typical result:

The dots and dashes are not clearly delineated ! In fact a swing up is a dot and it stays up if the next bit is a dot. A dash is a swing down. Note that 's' (...) above involves a long upward hump and 'o' (---) a downward one. I can only say it must have taken a lot of practice to read signals like the above.

The example below at least helpfully has the letter joins marked.

Nonetheless Bright was able to say in 1898 that "it requires only one ordinary clerk [!] at the receiving end, instead of a first rate mirror clerk and a writer". And, of course any ambiguities could be checked later by examining the tape in detail.

It will be apparent that the keying used to send the message was not the single action we are used to from WW2 films, with dash - dot Morse of long and short bleeps. At this time current flowing one way was a dot and the other way was a dash. This involved using two keys next to each other.



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