What is a Galvanometer ? How does it work ?

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.galvanometer schematic

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