Chapter-4|Moving Charges and Magnetism| NCERT 12th Physics:
Moving Coil Galvanometer:
Galvanometer is used for detecting and measuring small electric currents.
Its working is based on the fact that when a current carrying coil is placed in a magnetic field, it experiences a torque.
The moving coil galvanometer consists of a coil with a many turns free to rotate about a fixed axis, in a uniform radial magnetic field. There is a cylindrical soft iron core which not only makes the field radial but also increases the strength of the magnetic field.
When a current flows through the coil, a torque ats on it.
As terminal are connected (switch on) current flows in the coil and coil start rotating due to torque on it.
If N be the number of turns of the coil with the area A enclosed by a coil, B is the strength of magnetic field, and theta be the angle between coil and magnetic field, then