## The torque on a current loop in a uniform magnetic field is used to measure electrical magnetic field is used to measure electrical currents.  This current measuring device is called a moving coil galvanometer.  The galvanometer consists of a coil of wire often rectangular, carrying the current to be measured.  There are generally many turns in the coil to increase its sensitivity.  The coil is placed in a magnetic field such that the lines of B remain nearly parallel to the plane of wire as it turns.  This is achieved by having a soft iron cylinder placed at the center of the coil.  Magnetic field lines tend to pass through the iron cylinder, producing the field configuration. The moving coil is hung from a spring which winds up as the coil rotates; this winding up produces a restoring torque proportional to the winding up (or twisting) of the spring, i.e. to the angular deflection of the coil.  The coil comes to equilibrium when this restoring torque k balances the torque due to the magnetic field balances the torque due to the magnetic field.  Since by design field lines are radial,

we have  sin q ~ 1, so that for equilibrium

k ø = INBA

NBA

ø = ------ I

k

Thus the deflection  ø of the galvanometer is proportional to the electric current I passing through it.    