(Lord Kelvin) Thomson's mirror galvanometer (land type) used at Valentia Island end of the original Atlantic cable, made by White and Barr, Glasgow, Scotland, 1858.
A mirror galvanometer is a particularly sensitive electrical indicating instrument and was used to detect the extremely weak currents received through long submarine cables. Instead of a needle indicator, a mirror with a bar magnet cemented on the back is suspended on silk threads within the coil. A beam of light shone on the mirror is reflected on to a scale card some distance away, thus magnifying any movement of the mirror. This instrument was used at the Valentia Island end of the 1858 transatlantic telegraph cable to receive signals from Newfoundland. Unfortunately, the cable failed after less than a month. The mirror galvanometer became the standard instrument for receiving telegraph messages, but it did have a disadvantage in that it needed two operators - one to read and call the signal, and another to write the message down. This problem was solved by Thomson when he invented the siphon recorder, which used a glass tube dipped ink to record to movements of the cable on a moving tape, to then be read and decoded.