De Sauty Bridge measures an unknown capacitance in term of a standard capacitance i.e. comparing two capacitance’s Two ratio arm of this bridge consist pure resistor and two consist capacitor where one is of known value and another is standard capacitor.
C1 = Capacitor whose capacitance is to be measured
C2 = a standard capacitor
R1, R2 = non-inductive resistors
Balance is obtained by varying either R1 or R2. For balance, point B and D are at the same potential.
I1R1 = I2R2 and (–j/ωC1).I1 = (-j/ωC2).I2
Dividing one equation by the other, we get
R1/R2 = C1/C2; C1 = C2R1/R2
The bridge has maximum sensitivity when C1=C2. The simplicity of this method is offset by the impossibility of obtaining a perfect balance if both the capacitors are not free from the dielectric loss. A perfect balance can only be obtained if air capacitors are used.