Null-type, d.c. bridge (Wheatstone bridge)
A null-type bridge with d.c. excitation, commonly known as a Wheatstone bridge, has the form shown in Figure 7.1. The four arms of the bridge consist of the unknown resistance Ru, two equal value resistors R2 and R3 and a variable resistor Rv (usually a decade resistance box). A d.c. voltage Vi is applied across the points AC and the resistance Rv is varied until the voltage measured across points BD is zero. This null point is usually measured with a high sensitivity galvanometer.
To analyses the Whetstone bridge, define the current flowing in each arm to be I1 . . . I4 as shown in Figure 7.1. Normally, if a high impedance voltage-measuring instrument is used, the current Im drawn by the measuring instrument will be very small and can be approximated to zero. If this assumption is made, then, for Im D 0:
I1 =I3 and I2 =I4
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