A galvanometer is a very basic device which measures the current passing through it. It consists of a coil placed in a magnetic field. When a current passes through the coil, it experiences a deflection proportional to the charge through it. If a spring is used to restrain the coil, the deflection can be used to measure the current or a voltage.
The galvanometer is a simple instrument. The principle of the galvanometer is used in ammeters as well as voltmeters.
The galvanometer can be used as an ammeter by connecting a shunt in parallel to the coil. If the shunt is connected in series with the galvanometer coil, the galvanometer becomes a voltmeter.
See also Mirror Galvanometer