2dix-The Student Choice
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To measure the unknown capacitance by De-sauty bridge.


OBJECT: To measure the unknown capacitance by De-sauty bridge.



Two Decade Capacitors of  0.1µF total 1µF , An impedance Head Phone , Detecting cables & Bridge Oscillator OMEGA TYPE BO-316.



Looking at the bridge circuit as drawn on the panel of this ECB we see that two arms of  bridge are made up of resistances where as the other arms contain capacitors. Now as we apply an alternating signal (A.C.) from an Oscillator to the two points formed by junction of two resistors and by junction of two capacitors. The capacitors offer some opposition to the applied alternating signal. From A.C theory we know that this opposition is called the capacitive reactance denoted by Xc .the capacitive reactance is given by –

Xc = 1/(2πfc)

Where,  π  =  3.14

               f = frequency of applied signal from the oscillator.

               C = capacitance of the capacitor

When the bridge is balanced then the voltage at the junctions of each resistance and a capacitor i.e., at two green sockets is same and hence no current flows through the headphone connected between these two sockets. In this condition we can say that, R1/R2 = XC1/XC2

Where Xc1/Xc2 are reactance’s of two capacitors C1 & C2 respectively .Now, by substituting, we get - R1/R2 = C2/C1. Thus if  the value of capacitor C2 is unknown then it can be found as –  C2 = C1*(R1/R2)



Fig. 11.1 De-sauty bridge


  1. Connect the oscillator between two sockets Red and Black marked with the symbol of an oscillator between them. Connect the head phone between two green sockets with the head phone symbol marked between them.
  2. Keep out the output value of oscillator to a low level and switch ON the Oscillator.
  3. Select values of C1 and C2 at random.
  4. Now select another value of R1 and keep it constant, again vary R2 till sound from headphones is minimum. Note this second ratio of R1/R2 in table 1. Take a few such readings and find the mean value of R1/R2 ratio.



Sr. No.






      ( µF )

C2 = C1*(R1/R2)

( µF )

















      Here C1 is known & C2 is unknown                 [C2/C1 = Mean R1/R2]

RESULT: The unknown capacitance value is…………..

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