With the help of a neat labelled diagram expain MCG and obtain an expression for current sensitivity of MCG.

Explanation

NS - poles of powerful horseshoe magnet
FMS - annular spring
P - pointer
JB - jewel bearing
C - coil
S - linear scale
FL - field lines
we know that torque acting on the coil
`tau=NIAB-` (This is also known as the deflecting couple)
Restoring couple is due to the spring with couple per unit twist k.
If `phi` is the deflection in radian then at equilibrium
Restoring couple = deflecting couple
`Kphi=NIAB`
Hence, `" "theta=(NIAB)/K`
Then ratio `theta/I=(NAB)/K` is known as the current sensitivity. If R is the electrical resistance of the moving coil then V = IR or `I=V/R`
Hence, `" "theta/V=((NAB)/K)1/R " where " (theta/V)` is known as the voltage sensitivity.