Bridges for Measuring R, L and C

Wheatstone bridge at balance (no galvanometer current):

$\frac{R_1}{R_2} = \frac{R_3}{R_x} \;\Rightarrow\; R_x = \frac{R_2 R_3}{R_1}$

Used for medium resistances (1 Ω – 1 MΩ). Others:

• Kelvin double bridge — low resistances (< 1 Ω), eliminating lead resistance.
• Maxwell bridge — inductance (medium Q coils).
• Hay bridge — high-Q inductance.
• Schering bridge — capacitance and dielectric loss (tan δ) of cables and insulation.
• Wien bridge — frequency.

Today, LCR meters automate these measurements, but bridges remain the principle behind strain-gauge sensors.