De Sauty’s Bridge

This is a simple method for comparing two capacitances. C₁ and C₂ are the two capacitances to be compared. R₁ and R₂ are variable, non-inductive resistances. Audio frequency oscillator is connected between A and C, and headphone between B and D.

When the bridge is balanced,
    Z₁/Z₂ = Z₃/Z₄ or Z₁/Z₃ = Z₂/Z₄

Here, Z₁ = R₁; Z₂ = 1/jωC, Z₃ = = R₂, Z₄ = 1/jωC₂

. : R₁/R₂ = 1/jωC₁ / 1/jωC        or    C₂/C₁ = R₁/R₂

. : C₂ = C₁R₁/R₂                                            … (1)

If C₁ is known, C₂ can be calculated.

Eq. (1) is the only equations to be satisfied at balance. Hence R₁ and R, are suitably varied until the sound in the headphone is inaudible.