A mirror galvanometer is a mechanical meter that senses electric current, except that instead of moving a needle, it moves a mirror. The mirror reflects a beam of light, which projects onto a meter, and acts as a long, weightless, massless pointer.
The apparatus is also known as a spot galvanometer after the spot of light produced in some models.
Mirror galvanometers were used extensively in scientific instruments before reliable, stable electronic amplifiers were available. The most common uses were as recording equipment for seismometers and submarine cables used for telegraphy.
The mirror galvanometer was invented by William Thomson, later to become Lord Kelvin.