Sensitivity: 

• It is defined as the ratio of the changes in the output of an instrument to a change in the value of the quantity being measured.
• It denotes the smallest change in the measured variable to which the instrument responds.

\(\rm(Sensitivity\left( S \right) = \frac{{change{\rm{ }}in{\rm{ }}output}}{{unit{\rm{ }}change{\rm{ }}in{\rm{ }}input}} = \frac{{{\rm{\Delta }}output}}{{{\rm{\Delta input}}}})\)

• The sensitivity of an instrument is determined by dividing the sum of the resistance of the meter (Rm) and the series resistance (Rs), by the full-scale reading in volts.

Mathematically, sensitivity is expressed as:

\(\rm Sensitivity=\frac{R_m+R_s}{E}\)

Expressing the above expression in units, we get:

\(\rm Sensitivity=\frac{Ohms}{Volt}\)

Unit of instrument sensitivity is expressed in Ohm/Volt

Sensitivity is also expressed as:

\(\rm Sensitivity=\frac{1}{ampere}\)

So, sensitivity is said to be equal to the reciprocal of the full-scale deflection current.