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Quarter-Bridge Circuit
The schematic diagram shown below illustrates a representative situation in which an active gage, in a three-wire circuit, is remote from the instrument and connected to it by leadwires of resistance .

   

Quarter-bridge circuit with active gage remote from instrument.

If all leadwire resistances are nominally equal, then and ; i.e., the same amount of leadwire resistance is in series with both the active gage and the dummy. There is also leadwire resistance in the bridge output connection to the S- instrument terminal. The latter resistance has no effect, however, since the input impedance of the instrument applied across the output terminals of the bridge circuit is taken to be infinite. Thus, no current flows through the instrument leads.

To calibrate in compression, the active gage is shunted by a calibration resistor calculated from Eq. ( 514.7 ) or selected from Table 1 for the specified strain magnitude. After adjusting the sensitivity of the instrument to register the calibration strain, the effect of the leadwire resistance is eliminated from all subsequent strain measurements.

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