A Galvanometer directly measures the current (flow of electric charges) through a wire. It is a small pivoting coil of wire in the field of a permanent magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny spring pulls the coil and pointer to the zero position. In some meters, the magnetic field acts on a small piece of iron to perform the same effect as a spring.
When a current flows through the coil, the coil generates a magnetic field. This field acts with or against the permanent magnet. The coil pivots, pushing against the spring, and moving the pointer. The hand points at a scale indicating the electric current.
Some scales have mirrors next to the markings. The reflection of the hand is more accurate, because there is less parallax error.
Extremely sensitive equipment once used mirror galvanometers that substituted a mirror for the pointer. A beam of light reflected form the mirror acted as a long, massless pointer.
Galvanometers have been replaced by analog to digital converters for most uses. The term "galvanometer" derives from the name of Luigi Galvani.