Voltmeter is an instrument used to measure potential difference between the two ends of a current carrying conductor. A galvanometer is a type of sensitive ammeter or digital multimeter for detecting electric current. Galvanometers were the first instruments used to detect and measure electric currents. A galvanometer can be converted in to a voltmeter by connecting a high resistance in series connection within it. The scale is calibrated in volt. The value of the resistance connected in series decides the range of the voltmeter.
Galvanometer resistance =G
The current required to produce full scale Deflection in the galvanometer =Ig
Range of voltmeters =V
Resistance of voltmeter =R
Since R is connected in series with the galvanometer, the current through the galvanometer,
∴ R=V/Ig −G
The resistance is calculated by this equation which is connected in series. The effective resistance of the voltmeter is Rv=G+R. Rv is very large and hence a voltmeter is connected in parallel in a circuit as it draws the least current from the circuit.