GATE 2021 IN – Question 62
The bridge voltmeter reads zero with $R_1=100\ \Omega$, $R_2=110\ \Omega$, $R_3=90\ \Omega$, and $R_1/R_2=R_A/R_B$. The value of $R_4$ is ______ $\Omega$.
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Correct answer: 99 to 99
Explanation
The matching ratios R1/R2=RA/RB eliminate the interconnection-resistance term, including the displayed 1 Ω. The Kelvin balance reduces to R1/R2=R3/R4.
Hence R4=R3R2/R1=90·110/100=**99 Ω**. Adding 1 Ω would defeat the purpose of the compensating ratio arms.