A deflection-type bridge with d.c. excitation is shown in Figure . This differs from the Wheatstone bridge mainly in that the variable resistance Rv is replaced by a fixed resistance R1 of the same value as the nominal value of the unknown resistance Ru .

**Deflection-type d.c. bridge**

A
deflection-type bridge with d.c. excitation is shown in Figure . This
differs from the Wheatstone bridge mainly in that the variable resistance R_{v}
is replaced by a fixed resistance R_{1} of the same value as the
nominal value of the unknown resistance R_{u} . As the resistance R_{u}
changes, so the output voltage V_{0} varies, and this relationship
between V_{0} and Ru must be calculated.

This
relationship is simplified if we again assume that a high impedance voltage
measuring instrument is used and the current drawn by it, Im , can be
approximated to zero. (The case when this assumption does not hold is covered
later in this section.) The analysis is then exactly the same as for the
preceding example of the Wheatstone bridge, except that R_{v} is
replaced by R_{1}. Thus, from equation (7.1), we have:

V_{0}=
V_{i} * ( R_{u} / R_{u} + R_{3})- ( R_{1} / R_{1}+ R_{2})

When R_{u}
is at its nominal value, i.e. for R_{u} D R1, it is clear that V_{0}
D_{0} (since R_{2} D R_{3}). For other values of Ru, V_{0}
has negative and positive values that vary in a non-linear way with R_{u}.

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Measurements and Instrumentation : Comparison Methods of Measurements : Deflection-type D.C. bridge |

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