Consider a circuit as shown below.
Now, if R>>15 then 15/R tends to 0 and (2+(15/R)) tends to
2 and therefore voltmeter reading tends to 50V.
If R<<15 then 15/R tends to infinity and (2+(15/R)) tends
to infinity and therefore voltmeter reading tends to 0V.
So, depending upon the resistance value of the voltmeter with
respect to resistance R2 , voltmeter can show 0V to 50V.
It is therefore concluded that if the resistance of the
voltmeter used is not large enough as compared to the resistance of the circuit
or element across which it is connected, then, the voltmeter itself acts as a
load for the circuit (hence called loading
effect of voltmeter) and the reading shown is lower than the actual.
Therefore, to minimise the loading effect, a voltmeter with
resistance value as high as possible with respect to the circuit resistance, is
used.
This also explains why a
voltmeter has high resistance.
Related :-
How to connect a wattmeter Related :-
Moving Iron Instruments - Attraction type
Dynamometer type Wattmeter theory
