Here we measure in unknown capacitance in terms of known capacitance and known resistance. The device gives the expression of C3 in terms of R1, R4, and C2. Construction of De Sauty Bridge For showing the basic construction of a De Sauty Bridge let us draw the circuit diagram of such bridge. The second arm that is arm BC consists of a capacitor of unknown capacitance C2. Then the third arm that is arm CD consists of a standard capacitor of known capacitance C3. Forth arm that is BA consists of a pure resistance R4.
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This bridge measured an unknown capacitance in terms of a standard capacitance of capacitor i. Comparing two capacitances to ratio arms of this bridge. In which, one arm of the bridge consists of pure resistance and other arm two capacitors where one is known capacitance and another is standard capacitance.
The arm consists of capacitor C1 whose value is un known which carries current i1as shown, arm consist of pure resistor here pure resistor means we assume it non-inductive in nature , arm also consists of pure resistor and arm consist of standard capacitor whose value is already known to us. Balance is obtained by varying either R3 or R4 for balance, point 1 and 2 are at same potential. Where, C1 is the unknown capacitor. In order to obtain the balance, point we must adjust the values of either R3 or R4 without disturbing any other element of the bridge.
This is the most efficient method of comparing the two values of capacitor if all the dielectric losses neglected from the circuit. Phasor diagram: The phasor diagram of de sandy bridge is shown in figure Let, The voltage drops across unknown capacitor be E1 Voltage drop across the resistance R3 be E3 Voltage drop across arm be E4 Voltage drop across arm be E2 At the balance condition the current flows through path will be zero and also voltage drops E1 and E3 be equal to voltage drops E2 and E4 respectively.
In order to draw the phasor diagram E3 or E4 are taken as reference axis, E1 and E2 are shown at right angle to E1 or E2. Obtain square wave output, of amplitude of 9v and frequency of 1khz from the bridge oscillator. Connect the de-sauty bridge kit by the help of path cords and other apparatus as descripted in circuit diagram.
Connect head phone detector to the circuit of detection of the balanced condition of the bridge. Vary the resistance R3 and R4 and capacitor C2 and observe the output sound from the headphone detector. If the output sound from the head phone detector is very low, then that very condition is considered to be balanced condition of the bridge.
Else the bridge is considered to be at unstable condition. Note down three points of unstable condition by varying the R3 and R4 and capacitor C2.
De Sauty Bridge Construction Circuit and Theory
Experimentation with De Sauty`s and Schearing Bridge