Question

A galvanometer may be converted into ammeter or voltmeter. In which of the following cases the resistance of the device will be the largest ? (Assume maximum range of galvanometer $=1\text{mA}$)

• A. an ammeter of range $10\text{A}$
• B. a voltmeter of range $5\text{V}$
• C. an ammeter of range $5\text{A}$
• D. a voltmeter of range $10\text{V}$

Answer 1

The correct option is D a voltmeter of range $10\text{V}$

Resistance of the device would be largest for the case of Voltmeter.
$V=i_g(R+r_g)$
Device resistance is $R_x=R+r_g$
Given $i_g=1\times {10}^{-3}\text{mA}$
$V=i_g\times R_x=1\times {10}^{-3}\times R_x$​
$R_x=1000\text{V}$
Maximum value will correspond to voltmeter of reading $10\text{V}$