(a) Draw a labelled diagram of a moving coil galvanometer. Describe briefly its principle and working.

(b) Answer the following:

(i) Why is it necessary to introduce a cylindrical soft iron core inside the coil of a galvanometer?

(ii) Increasing the current sensitivity of a galvanometer may not necessarily increase its voltage sensitivity. Explain, giving reason.

b) Moving coil galvanometer:
  
Principle: The underlying principle of moving coil galvanometer is that a current carrying coil, placed in a uniform magnetic field, experiences torque.

Consider a rectangular coil for which no. of turns = N

Are of cross-section is A = lb

Intensity of the uniform magnetic field = B

Current through the coil = I

Therefore,

Deflecting torque is given by,

BIL x b = BIA

For N number of turns,

= NBIA

Restoring torque in the spring = k

Therefore, 
          

b) i) The soft iron coil in a galvanometer will make the field radial. Also, it increases the strength of the magnetic field.
ii) Current sensitivity in the galvanometer is given by, 
Voltage sensitivity in the galvanometer is given by, 

The above two equations imply that increasing the current sensitivity may not necessarily increase the voltage sensitivity.