How it works
The Wheatstone bridge measures resistance accurately from about 1 Ω to 100 kΩ; below 1 Ω, lead and contact resistances introduce serious errors, so the Kelvin double bridge uses a second set of ratio arms (p and q) to eliminate these. For AC measurements, the Maxwell bridge balances unknown inductance L = R2R3C1 and Q = ωR1C1; valid for Q between 1 and 10. Schering bridge measures capacitance and dissipation factor: Cx = C1R4/R3, tan δ = ωC1R1. Hay bridge suits high-Q coils (Q > 10) while Maxwell suits low-to-medium Q.
Key points to remember
Bridge sensitivity S = ΔVout/ΔR — higher for smaller ratio arm values when supply voltage is fixed. The null condition (galvanometer zero) for all four bridges follows the same rule: products of opposite arms are equal. Anderson bridge is preferred for precise inductance measurement at low frequencies because it uses a capacitor and requires only DC null detection. Owen bridge measures inductance in terms of capacitance and resistance and is particularly suited to measurements at audio frequencies. Examiners test both the balance conditions and the derivation from phasor analysis.
Exam tip
Every Anna University instrumentation paper has a question asking you to derive the balance condition of a Maxwell bridge from first principles — set Vb = Vd at balance and equate real and imaginary parts of the admittance equation.