Definition: The Wagner earthing device is used for removing the earth capacitance from the bridges. It is a type of voltage divider circuit used to reduces the error which occurs because of stray capacitance. The Wagner Earth device provides high accuracy to the bridge.
At high frequency, stray capacitance is induced between the bridge elements, ground and between the arms of the bridge. This stray element causes the error in the measurement. One of the way, of controlling these capacitances is too enclosed the bridge elements into the shield. Another way of eliminating these stray capacitance is to places the Wagner Earth device between the elements of the bridge.
Construction of Wagner Earthing Device
The circuit diagram of the Wagner Earth Device is shown in the figure below. Consider the Z1, Z2, Z3, and Z4 are the impedances arm of the bridge. The Z5 and the Z6 are the two variable impedances of the Wagner Earth Device. The centre point of the Wagner earth device is earthed. The impedance of Wagner device arms is similar to the arms of the bridge. The impedance of the arm consists the resistance and capacitances.
The Wagner impedance placed in such a way so that they make the bridge balance with Z1, Z3 and Z2, Z4. The C1, C2 C3 and C4 show the stray capacitances of the bridges. The D is the detector of the bridge.
The bridge comes in the balance condition by adjusting the impedances of arms Z1 and Z4. The stray capacitance prevents bridge to comes in the balanced condition. When the S is not thrown on ‘e’ then the D is connected between the point p and q. But when S is thrown on ‘e’ then the detector D is connected between the terminal b and earth.
The impedance Z4 and Z5 are adjusted until the minimum sound is obtained. The headphones again connected between the point b and d for obtaining the minimum sound. The headphones are reconnected between the point b and d, Z4 and Z5 are adjusted for obtaining the minimum sound. The process is continuously repeated for obtaining the silent sound.
The point b, d, and e all are in the same potential. And the capacitance C1, C2, C3, C4 all are eliminated from the bridge circuit along with the impedances Z5 and Z6.