A mirror galvanometer is an ammeter that indicates it has sensed an electric current by deflecting a light beam with a mirror. The beam of light projected on a scale acts as a long massless pointer. In 1826, Johann Christian Poggendorff developed the mirror galvanometer for detecting electric currents. The apparatus is also known as a spot galvanometer after the spot of light produced in some models.
Thomson mirror galvanometer of tripod type, from around 1900
Galvanometer by H.W. Sullivan, London. Late 19th or early 20th century. This galvanometer was used at the transatlantic cable station, Halifax, NS, Canada
Modern mirror galvanometer from Scanlab
A moving coil galvanometer with its glass cover separated and mirror absent
Johann Christian Poggendorff
Johann Christian Poggendorff, was a German physicist born in Hamburg. By far the greater and more important part of his work related to electricity and magnetism. Poggendorff is known for his electrostatic motor which is analogous to Wilhelm Holtz's electrostatic machine. In 1841 he described the use of the potentiometer for measurement of electrical potentials without current draw.
Johann Christian Poggendorff
Biographisch-literarisches Handwörterbuch zur Geschichte der exakten Wissenschaften, 1863