A galvanometer is an instrument used to show the direction and strength of the current passing through it. In a galvanometer, a coil placed in a magnetic field experiences a torque and hence gets deflected when a current passes through it. The name is derived from the surname of Italian scientist L. Galvani, who in 1791 discovered that electric current makes a dead frog's leg jerk. A spring attached with the coil provides a counter torque.\nIn equilibrium, the deflecting torque is balanced by the restoring torque of the spring and we have :\n$\text{NBAI} = \text{k}\phi$\nwhere N is the total number of turns in the coil\nA is the area of cross-section of each turn\nB is the radial magnetic field\nk is the torsional constant of the spring\n$\phi$ is the angular deflection of the coil