Wednesday, 27 April 2016

Conversion of galvanometer into a voltmeter

Voltmeter   is   an   instrument used  to  measure  potential  difference between  the  two ends  of  a current carrying  conductor.

A  galvanometer      can      be converted    into    a    voltmeter    by connecting   a   high   resistance   in series with 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  voltmeter  =  V
Resistance  to  be  connected  in  series  =  R
Since R is connected in series with the galvanometer, the current through  the  galvanometer,

Ig =  V/(R + G)

∴R  =  (V/Ig )–  G

From  the  equation  the  resistance  to  be  connected  in  series  with the  galvanometer  is calculated.
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.
The  resistance  of  the  voltmeter  should  be  very  large  compared  to  the resistance  across  which  the  voltmeter  is  connected  to  measure  the potential difference. Otherwise, the voltmeter will draw a large current from  the  circuit  and  hence  the  current  through  the  remaining  part  of the circuit decreases. In such a case the potential difference measured by the voltmeter is very much less than the actual potential difference.The error is eliminated only when the voltmeter has a high resistance.
An  ideal  voltmeter  is  one  which  has  infinite  resistance