Since the effect of leadwire resistance on bridge circuit nonlinearity is normally very small, terms involving have been omitted from Eq. ( 514.18 ). If the leadwire resistance is a significant fraction of the gage resistance, however, Eq. (514.18) tends to overcorrect for the nonlinearity. In such cases, the following complete relationship can be used to obtain more accurate correction:

    (514.18a)

Shunting the dummy arm of the bridge produces an upscale signal, and can be used to simulate a tensile strain in the active gage. For the simulation to be exact, however, a special shunt-calibration relationship is required, because the nonlinearity in tension is different from that in compression.

If the active gage were subjected to an actual tensile strain, the resistance of the right-hand branch of the bridge shown above would rise, and the current would decrease correspondingly. However, when the dummy arm of the bridge is shunted, the resistance of the branch decreases, and the current rises. This difference can be accounted for by calculating the calibration resistor so that the bridge output voltage due to shunting the dummy is the same as that for a preselected tensile calibration strain in the active gage. The procedure for doing so is demonstrated by the following derivation where, for the sake of simplicity, the effect of leadwire resistance is temporarily ignored ( = 0).

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