Side-by-side comparison
| Parameter | AC Bridge | DC Bridge |
|---|---|---|
| Excitation | DC voltage source (battery or regulated DC supply) | AC sinusoidal source, typically 1 kHz for audio bridges |
| Measured Components | Resistance only (R) | Resistance, inductance (L), capacitance (C), and Q or dissipation factor D |
| Balance Condition | Single condition: R1/R2 = R3/R4 (real numbers) | Two conditions: magnitude balance and phase angle balance simultaneously |
| Null Detector | Galvanometer (DC current meter) | Headphones (at 1 kHz), vibration galvanometer, or oscilloscope |
| Stray Capacitance Effect | No effect at DC (capacitors open) | Significant — stray capacitance between arms causes errors; Wagner earth eliminates it |
| Frequency Effect | None — DC, frequency irrelevant | Balance condition is frequency-dependent for most AC bridges |
| Example Bridge Types | Wheatstone (R), Kelvin double (low R), Murray (cable fault) | Maxwell (L, high Q), Hay (L, low Q), Schering (C, high voltage), Wien (frequency) |
| Typical Accuracy | 0.01% for precision Wheatstone; 0.001% for Kelvin bridge | 0.1–1% for LCR measurements; 0.1% for Schering bridge capacitance |
Key differences
DC bridge balance requires one condition: P/Q = R/S. AC bridge balance requires two simultaneous conditions — the real parts must balance and the imaginary parts must balance independently. For a Maxwell bridge measuring inductance: balance gives L_x = R2 × R3 × C and R_x = R2 × R3 / R4, two equations solved by alternately adjusting R4 (for magnitude) and C (for phase). Stray capacitance between bridge arms — irrelevant at DC — causes AC bridges to fail to achieve null unless a Wagner earth network is added to drain stray currents. Wien bridge specifically measures frequency: f = 1/(2πRC), which is how the HP 200A audio oscillator (1939) generated stable low-distortion sine waves.
When to use AC Bridge
Use a DC bridge (Wheatstone) when measuring resistance — for example, using a Kelvin double bridge to measure a 0.001 Ω precision resistor with 0.005% accuracy by eliminating lead and contact resistance.
When to use DC Bridge
Use an AC bridge when measuring inductance, capacitance, or dissipation factor — for example, using a Schering bridge at 1 kHz to measure the capacitance and tan δ of a 100 nF high-voltage capacitor to verify insulation quality.
Recommendation
For exam problems, identify the component to be measured first. Resistance at DC means Wheatstone or Kelvin bridge. Inductance means Maxwell or Hay bridge (choose Maxwell for high Q, Hay for low Q). Capacitance means Schering bridge. That component-to-bridge mapping resolves every bridge-selection question.
Exam tip: Examiners ask why AC bridge balance requires two conditions — state that a complex equation Z1 × Z4 = Z2 × Z3 has both real and imaginary parts that must each independently equal zero, giving two simultaneous equations for two unknowns.
Interview tip: Interviewers at test and measurement companies like Keysight or Hioki ask about the Wagner earth in AC bridges — explain that the Wagner earth is an auxiliary balancing network connected between the bridge midpoints and ground that drains stray capacitance currents, preventing them from unbalancing the main bridge arms.