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If $R_{1}=R_{2}=R_{4}=R$ and $R_{3}=1.1 R$ in the bridge circuit shown in the figure, then the reading in the ideal voltmeter connected between $a$ and $b$ is

1. $0.238 \mathrm{~V}$.
2. $0.138 \mathrm{~V}$.
3. $-0.238 \mathrm{~V}$
4. $1 \mathrm{~V}$.