Current and Voltage Sensitivities of a moving-coil galvanometer

The figure of merit or current sensitivity (Sc) of a moving coil mirror galvanometer is the current that is required to produce a deflection of 1mm on a scale kept at a distance of 1 meter from the mirror.
It is expressed in μA/mm.
The voltage sensitivity (Sv) is the p.d. that should be applied to the galvanometer to produce a deflection of 1mm on a scale at a distance of 1meter.

It is expressed in μ V/mm.
To determine the current and voltage sensitivities of a galvanometer. Two resistance boxes P and Q and a key K are connected in series with a lead accumulator of emf E. Between the ends of P, a resistance box R and the M.G., through a commutator, are connected.

A low resistance is infroduced in P and high resistance in Q. With no resistance in R, the deflection θ of the galvanometer is found.
    . : Voltage sensitivity, Sv = _____EP_____ x 106 μ V/mm
The resistance in R is adjusted such that the deflection becomes θ/2.
The resistance in R is equal to the galvanometer resistance Rg.

. : Current sensitivity Sc = _____EP_____  x 106 μ A/mm
The experiment is repeated for various of P keeping P + Q constant.





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