A galvanometer is an instrument used to detect and measure electrical current.[1]​ It is an electromechanical analog transducer that produces a rotational deformation in a needle or pointer in response to electrical current flowing through its coil.[2]​ This term has been expanded to include uses of the same device in recording equipment, positioning, and servomechanisms.

It is capable of detecting the presence of small currents in a closed circuit, and can be adapted, through calibration, to measure their magnitude. Its operating principle (moving coil and fixed magnet) is known as the D'Arsonval mechanism,[2]​ in honor of the scientist who developed it. This consists of a normally rectangular coil, through which the current to be measured circulates. This coil is suspended within the magnetic field associated with a permanent magnet, along its vertical axis, in such a way that the angle of rotation of said coil is proportional to the current that passes through it.[3]​ The vast majority of needle indicator instruments used in analog instruments are based on the operating principle explained, using a coil suspended within the field associated with a permanent magnet. The suspension methods used vary, which determines the sensitivity of the instrument. Thus, when the suspension is achieved by a taut metal tape, full scale deflection can be obtained with only 2 μA, but the instrument is extremely fragile, while the "jewels and pivots" system, similar to that used in watchmaking, allows obtaining a more robust but less sensitive instrument than the previous one, in which full scale deflection is typically obtained, with 50 μA.

The deflection of the needles of a magnetic compass by current in a wire was first described by Hans Oersted in 1820.[4] The first galvanometers were described by Johann Schweigger at the University of Halle on September 16 of that year. The French physicist André-Marie Ampère also contributed to its development. Early designs increased the effect of the magnetic field due to current by using multiple turns of wire; These instruments were called "multipliers" because of this common design feature. The term "galvanometer", in common use since 1836, is derived from the surname of the Italian researcher Luigi Galvani, who discovered that electric current could make a frog's leg move.[5]​.

Originally, galvanometers relied on the Earth's magnetic field to provide the force to reset the compass needle; These were called "tangent" galvanometers and had to be oriented, according to the Earth's magnetic field, before use.[6] Later, "static" type instruments used opposing magnets, which made them independent of the Earth's magnetic field and could operate in any orientation. The most sensitive form, the Thompson or mirror galvanometer, was invented by William Thomson (Lord Kelvin). Instead of having a needle, it used tiny magnets attached to a small light mirror, suspended by a thread. It was based on the deflection of a highly magnified beam of light due to small currents. Alternatively, the deflection of the suspended magnets could be observed directly through a microscope.