Question

In Maxwell’s Bridge, as shown in the figure below, the values of the resistance R1 and inductance L1 of a coil are to be calculated after the bridge is balanced. The values are?

(a) 375 Ω and 75 mH

(b) 75 Ω and 150 mH

(c) 37.5 Ω and 75 mH

(d) 75 Ω and 75 mH

This question was addressed to me during a job interview.

This intriguing question comes from Advanced Problems on Measurement of Inductance using AC Bridges in division Measurement of Inductance and Capacitance of Electrical Measurements

Answer 1

Correct option is (a) 375 Ω and 75 mH

The best I can explain: Applying the usual balance condition relation,

Z1 Z4 = Z2 Z3

We have, (R1 + jL1 ω) \(\frac{R_4/jωC_4}{R_4+1/jωC_4}\) = R2 R3

Or, R1 R4 + jL1 ωR4 = R2 R3 + j R2 R3 R4 C4 ω

∴ R1 = 2000 × \(\frac{750}{4000}\) = 375 Ω

∴ L1 = 2000 × 750 × 0.5 × 10^-6

= 75 mH.