Basic DC Voltmeter

The basic d.c. voltmeter is nothing but a PMMC D'Arsonoval galvanometer. The resistance is required to be connected in series with the basic meter to use it as a voltmeter.

This series resistance (Rs) is called a multiplier. The main function of the multiplier is to limit the current through the basic meter so that the meter current does not exceed the full scale deflection (Ifsd) value. The voltmeter measures the voltage across the two points of a circuit or a voltage across circuit component.

The multiplier resistance can be calculated as:

                                 From Figure,

                                  V = I_m  (R_m + R_s )

                                  V  =  I_mR_m + I_m   R_s 

                                              I_mR_s = V -  I_m  R_m

       Where,   R_m = Internal resistance of coil i.e. meter

                      R_s  = series multiplier resistance 

                      I_m  = full scale deflection current

                      V   = full range voltage to be measured

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