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Equation ( 514.1a ) is perfectly general in application to constant-voltage Wheatstone bridges, regardless of the values of , , , and . In conventional strain gage instrumentation, however, at least two of the bridge arms normally have the same (nominal) resistance; and all four arms are often the same. For simplicity in presentation, without a significant sacrifice in generality, the latter case, known as the "equal-arm bridge", is assumed in the following, and pictured below.

Shunt calibration of single active gage.

The diagram shows a single active gage, represented by , and an associated calibration resistor, , for shunting across the gage to produce an output signal simulating strain. The bridge is assumed to be in an initial state of resistive balance; and all leadwire resistances are assumed negligibly small for this introductory development of shunt-calibration theory. Methods of accounting for leadwire resistance (or eliminating its effects) are given in the section on Instrument Scaling for Small Strains .

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