# Moving coil Galvanometer

Moving coil Galvanometer:

(i)Principle of moving coil galvanometer: When a current carrying coil suspended in a uniform magnetic field, it experience a torque and hence it rotates.

(ii)Poles of magnet are concave is shape, to make the magnetic field radial so that at all orientations the plane of the coil is parallel to the field, and hence torque acting on it is maximum. This makes the relation between current and deflection linear.

(iii) Soft iron cylinder is kept at the center of magnetic field to increase the flux.

(iv)Phosphor Bronze has :

(a)High Young’s modulus so that the wire will not be stretched easily.

(b)Low rigidity modulus so that the wire can be twisted easily.

(c)Small elastic after effect so that it comes back quickly to original position after withdrawing current.

(v)Small mirror is attached on the phosphor Bronze wire, to measure the deflection using lamp and scale arrangement.

(vi)If ‘θ’ is the deflection for passes of current ‘I’, then

C θ = NIAB  ⇒ $\large I = \frac{C}{NBA}\theta$

where  C = Galvanometer constant or figure of merit.

It is independent of BH. Where ‘C’ is couple per unit twist.

(vii) (a) Current sensitivity of a galvanometer is defined as the deflection produced in the galvanometer

per unit current flowing through it.

$\large S_I = \frac{\theta}{I} = \frac{NBA}{C}$

(b)Voltage sensitivity of a galvanometer is defined as the deflection produced in the galvanometer per unit voltage applied to it.

$\large S_V = \frac{\theta}{V} = \frac{NBA}{C G}$ ; Where G is resistance of galvanometer

(viii)It is used to measure current upto a minimum of 10-9

(a)Plane of coil need not be along the magnetic meridian

(b)Galvanometer constant is independent of BH. So it can be used to measure currents even at poles.

(c)External magnetic fields have no effect on deflection. So, it can be used to measure current even in the environment of stray magnetic fields.