The capacitance can be measured using Schering bridge. It is a widely used bridge for the measurement of capacitance. This bridge though is used for measuring capacitance is very useful in measuring insulation properties i.e. for phase angles very near to 90°. The schematic representation to this bridge is shown in Figure. One of the ratio arms contains a parallel combination of capacitor and resistor C_{1} and R_{1}. The other ratio arm consists of a variable resistor R_{2}. The standard arm consists of a capacitance of fixed value C_{3}. Mica capacitor or well designed air dielectric type capacitor are used. The reason for the choice is the phase angle of the two having a value nearing 90°.

_{1}parallel with R

_{1}. Schering bridge formula is derived as follows

_{x}= Z

_{2}Z

_{3}Y

_{1}

_{x}–j/ωC

_{x}= R

_{2}(-j/ωC

_{3})(1/R

_{1}+ jωC

_{1})

_{x}–j/ωC

_{x }= R

_{2}C

_{1}/C

_{3}– jR

_{2}/ωC

_{3}R

_{1}--------------------------- 1

_{x}= R

_{2}C

_{1}/C

_{3}----------------------------------- 2

_{x}= C

_{3}R

_{1}/R

_{2}----------------------------- 3

_{1}and resistor R

_{2}. The dial of the capacitor C

_{2}can be calibrated in terms of dissipation factor D, by keeping the value of R

_{1}fixed. This can be explained as follows :

_{x}/Z

_{x}. For phase angles very close to 90°, the reactance is very nearly equal to the impedance and with a close approximation the power factor can be taken

_{x}/ X

_{x}= ωC

_{x}R

_{x}---------------------------- 4

_{x}/ X

_{x}= ωC

_{x}R

_{x}---------------------------------------- 5

_{L}/R

_{L}, the dissipation factor, D is the reciprocal of the quality factor, Q and therefore D = I/Q. If we substitute the value of C

_{x}in equation (3), and of R

_{x}in equation (2), into the expression for the dissipation factor we obtain.

_{1}C

_{1}-------------------------------- 6

_{1}in the bridge fixed, the dial of capacitor C

_{1}can be calibrated directly in dissipation factor D. One point to be noted here is that the term co appears in the equation for the dissipation factor D. Hence the calibration of the capacitor C

_{1}is valid for one frequency at which it is calibrated. When a different frequency is used a correction can be made by multiplying the C

_{1}dial reading by the ratio of the two frequencies. This is the advantage of using Schering bridge.

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