Galvanometer

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/galvan.html

Galvanometer

Galvanometer is the historical name given to a moving coil electric current detector. When a current is passed through a coil in a magnetic field, the coil experiences a torqueproportional to the current. If the coil's movement is opposed by a coil spring, then the amount of deflection of a needle attached to the coil may be proportional to the current passing through the coil. Such "meter movements" were at the heart of the moving coil meters such as voltmeters and ammetersuntil they were largely replaced with solid state meters.

The accuracy of moving coil meters is dependent upon having a uniform and constant magnetic field. The illustration shows one configuration of permanent magnet which was widely used in such meters.

http://www.ncert.nic.in/html/learning_basket/electricity/electricity/effects_of_current/galvanometer.htm  

Galvanometer

     

        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
.