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Metre wire bridge

A practical arrangement of the Wheatstone bridge

In this version the two ratio arms of the bridge are the two sections of a metre wire as shown in Figure 1. The circuit is shown in Figure 1(a) and a practical arrangement in Figure 1(b).

A standard resistance S is used and hence the value of the unknown resistance R can be found.

For this bridge the balance condition (no current in the galvanometer) is given by the equation:

Balance condition:     R = [L1/L2]S

Accuracy of a metre wire bridge

Measurement of resistance with a metre wire bridge Let the unknown resistor be R, the standard resistor be S and the two lengths of wire at the balance point be L1 and L2 .
Therefore the fractional error in R is given by:
DR/R = DS/ S +DL1/L1 + DL2/L2

Therefore the greatest accuracy is obtained when L1 = L2 for a given value of DL. The balance point must be in the centre of the wire.
© Keith Gibbs 2011