(Lord Kelvin) Thomson's marine mirror galvanometer, made by White & 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. Although he did not originate it, William Thomson, later Lord Kelvin (1824-1907) made improvements which he patented in 1858.