Wagner Ground Connection

SHIELDING AND GROUNDING OF BRIDGES: 

Stray capacitance exists between the various bridge elements and ground. It also exists between the bridge arms. These stray capacitances shunt the bridge arms and cause errors in the measurements. This is more particularly at the higher frequencies, while measuring small capacitances and large inductors. To eliminate the stray capacitance, shields are used. Shields are made of copper or aluminium and are grounded. The use of shields will not eliminate stray capacitances completely, but marks them constant in value such that they can be compensated.

WAGNER GROUND CONNECTION: 

This is an arrangement that eliminates the capacitance (stray) between the detector terminals and ground. The schematic diagram is shown for a capacitance bridge in Figure. The capacitors C₁ and C₂ represent the stray capacitances. The oscillator is removed from its usual ground connection and bridged by a series combination of resistor R_w and capacitor C_w. The junction of R_w and C_w is called Wagner ground connection and is connected to ground. Initially the detector is connected to point 1 and R₁ is adjusted for null or minimum balance (minimum sound in headphones). This is obtained by throwing the switch to position 1. Later the switch is set to position 2. It connects the detector to the Wagner ground point. Resistor R_w is now adjusted for minimum sound.

When the switch is set to position one again; there will be some unbalance. Resistances R₁ and R₃ are then adjusted for minimum detector response. Again the switch is set to position 2. When a null is obtained finally the points 1 and 2 are at the same potential which is ground potential. When this condition is achieved the stray capacitances C₁ and C₂ are effectively shorted and have no effect on normal bridge balance. There are also capacitances from points C and D to ground. These capacitances are eliminated by Wagner ground. With Wagner ground the capacitances of the bridge arms are not eliminated. Therefore care is to be taken to shield the bridge arms.