To measure the unknown capacitance by Desauty bridge.
OBJECT: To measure the unknown capacitance by Desauty bridge.
APPARATUS REQUIRED:
Two Decade Capacitors of 0.1µF total 1µF , An impedance Head Phone , Detecting cables & Bridge Oscillator OMEGA TYPE BO316.
THEORY:
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 X_{c} .the capacitive reactance is given by –
X_{c }= 1/(2πf_{c})
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, R_{1}/R_{2} = XC_{1}/XC_{2}
Where X_{c1}/X_{c2} are reactance’s of two capacitors C_{1} & C_{2} respectively .Now, by substituting, we get  R_{1}/R_{2} = C_{2}/C_{1}. Thus if the value of capacitor C_{2} is unknown then it can be found as – C_{2} = C_{1}*(R_{1}/R_{2})
CIRCUIT DIAGRAM:
Fig. 11.1 Desauty bridge
PROCEDURE:
 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.
 Keep out the output value of oscillator to a low level and switch ON the Oscillator.
 Select values of C_{1} and C_{2} at random.
 Now select another value of R_{1} and keep it constant, again vary R_{2} till sound from headphones is minimum. Note this second ratio of R_{1}/R_{2 }in table 1. Take a few such readings and find the mean value of R_{1}/R_{2} ratio.
OBSERVATION TABLE:
Sr. No. 
R_{1} Ohms 
R_{2} Ohms 
C_{1 } ( µF ) 
C_{2} = C_{1}*(R_{1}/R_{2}) ( µF ) 
1. 




2. 




3. 




Here C_{1} is known & C_{2} is unknown [C_{2}/C_{1 }= Mean R_{1}/R_{2}]
RESULT: The unknown capacitance value is…………..
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