Galvanometre

A galvanometer is an electrical device that detects electric current, ranging from micro to milli amperes.

It consists of two concave-shaped permanent magnets, providing a uniform radial magnetic field in the center.

A coil of fine copper wire wound on a soft iron core is placed between the permanent magnets. A needle and spring assembly is attached to the axis of this coil.

If no current flows through the coil, the needle points at the center of the scale.

When there is a current flow, the magnetic field exerts a torque on the coil; as a result, the coil turns.

The magnitude of torque is proportional to the current passed in the coil. When there is no current, the spring exerts a restoring torque.

Under equilibrium conditions, the angular deflection of the coil is obtained which is directly proportional to the coil current. 

The full-scale deflection is typically on the order of milli amperes.

The potential difference required for full deflection can be obtained using Ohm's law, where the coil resistance is typically ten to thousand ohms.