A mirror galvanometer is an electromechanical instrument that indicates that 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.
A galvanometer is an electromechanical instrument used for detecting and indicating an electric current . A galvanometer works as an actuator , by producing a rotary deflection (of a "pointer"), in response to electric current flowing through a coil in a constant magnetic field . Early galvanometers were not calibrated, but their later developments were used as measuring instruments, called ammeters , to measure the current flowing through an electric circuit.