Question

Wagner's earth devices are used in a.c. bridge circuits for

• A. Eliminating the effects of earth capacitance
• B. Eliminating the effects of inter component capacitance
• C. Eliminating the effects of stray electrostatic fields
• D. Shielding the bridge elements

Hint:

Wagner Earth Device. Used in AC bridges to eliminate errors caused by stray capacitances between bridge components (especially detector) and ground (earth). It helps achieve a true balance independent of these earth capacitances.

Answer 1

The Correct Option is A

In AC bridge measurements, especially at higher frequencies or when measuring small impedances/admittances, stray capacitances between bridge components (arms, detector, source) and earth (ground) can significantly affect the balance condition and measurement accuracy
The Wagner earthing device (or Wagner ground connection) is an auxiliary circuit added to the AC bridge
It consists of an additional impedance arm (typically a variable resistor and capacitor) connected across the AC source, with its midpoint connected to ground
By adjusting the Wagner arm components, the bridge detector terminals can be brought to the same potential with respect to earth potential *at balance*
This effectively eliminates the influence of stray capacitances between the detector terminals and earth on the main bridge balance condition
It specifically targets earth capacitances affecting the detector, not necessarily inter-component capacitances or general electrostatic fields, although minimizing detector-earth capacitance helps reduce sensitivity to stray fields