Principle: When current (I ) is passed in the coil, torque τ acts on the coil, given by τ = NIABsinθ
where θ is the angle between the normal to plane of coil and the magnetic field of strength B, N is the number of turns in a coil.
When the magnetic field is radial, as in the case of cylindrical pole pieces and soft iron core, then in every position of coil the plane of the coil, is parallel to the magnetic field lines, so that θ = 90° and sin 90° =1
Deflecting torque, τ = NIAB
A galvanometer cannot be used as such to measure current due to following two reasons.
(i) A galvanometer has a finite large resistance and is connected in series in the circuit, so it will increase the resistance of circuit and hence change the value of current in the circuit.
(ii) A galvanometer is a very sensitive device, it gives a full scale deflection for the current of the order of microampere, hence if connected as such it will not measure current of the order of ampere.
Current sensitivity of galvanometer depends on
(i) Number of turns N: It increases with increase of number of turns.
(ii) Area of coil A: It increases with increase of area of coil.
(iii) Strength of magnetic poles (B): It increases with increase of strength of poles.
(iv) Torsional rigidity of suspension: It increases with decrease of torsional rigidity of suspension.
