The signals arriving through a trans-Atlantic telegraph cable are very weak. The siphon recorder is an especially sensitive instrument based on a design by William Thomson (later Lord Kelvin) in 1867 for detecting these signals and recording them with ink on paper tape. It is based on a moving coil galvanometer with a powerful electromagnet, but this suspension is too delicate to carry a pen. The pen, which is a fine glass capillary with one end dipping in a reservoir of ink, is mounted on a separate suspension and coupled to the moving coil by silk threads. A vibrator on this suspension keeps the pen moving to overcome static friction. The glass capillary and a few other delicate parts of our instrument are missing.

Inscriptions / Translations:  MUIRHEAD & Co
WESTMINSTER
No 14,809
PATENT