A galvanometer is a type of ammeter: an instrument for detecting electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field.

Galvanometers were the first instruments used to detect and measure electric currents. Sensitive galvanometers were used to detect signals from long submarine cables, and were used to discover the electrical activity of the heart and brain. Some galvanometers used a solid pointer on a scale to show measurements, other very sensitive types used a tiny mirror and a beam of light to provide mechanical amplification of tiny signals. Initially a laboratory instrument relying on the Earth's own magnetic field to provide restoring force for the pointer, galvanometers were developed into compact, rugged, sensitive portable instruments that were essential to the development of electrotechnology. A type of galvanometer that permanently recorded measurements was the chart recorder. The term has expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.

Read more about Galvanometer:  History, Operation, Types