Function: A voltmeter is used to measure the potential difference across a circuit element and is always connected in parallel with that element .
Constraint: To measure the potential difference accurately, the voltmeter must not draw any current from the main circuit. If it draws current, it will alter the potential difference it is trying to measure.
Ideal Condition: An ideal voltmeter should draw zero current. According to Ohm's law (I=V/R), for the current (I) to be zero for a finite potential difference (V), the resistance (R) must be infinite.
Practical Realization: In practice, a galvanometer is converted into a voltmeter by connecting a very high resistance in series with it to approximate this ideal condition .
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