Question
What is being done to change the range of voltmeter?
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Answer
$R _{ H }=\left(\frac{ V }{i_g}\right)- G$.
On joining this high resistance to the given galvanometer in series, it will become voltmeter of range V . When high resistance $R _{ H }$ is joined with galvanometer in series, the effective resistance $R _{ V }=\left( R _{ H }+ G \right)$ of galvanometer becomes very high (camparison to G). So, when such a voltmeter is connected in parallel in a circuit to measure potential difference, then due to its high resistance, it allows very small fraction of total current to pass through it and hence gives reading of potential difference corresponding to total current flowing through the circuit.
On joining this high resistance to the given galvanometer in series, it will become voltmeter of range V . When high resistance $R _{ H }$ is joined with galvanometer in series, the effective resistance $R _{ V }=\left( R _{ H }+ G \right)$ of galvanometer becomes very high (camparison to G). So, when such a voltmeter is connected in parallel in a circuit to measure potential difference, then due to its high resistance, it allows very small fraction of total current to pass through it and hence gives reading of potential difference corresponding to total current flowing through the circuit.
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