Strain Gauge Used in a Bridge Circuit:

Strain gauges, which are nothing more than resistors, are used along with bridge circuits as display in Figure.

Figure: Strain Gauge Used in a Bridge Circuit

Instead current is given through an exciter which is used in place of a battery to eliminate the requirement for a galvanometer. While a change in resistance within the strain gauge causes an unbalanced condition, an error signal enters the amplifier and actuates the balancing motor.  A balancing motor moves the slider along the slidewire, restoring the bridge to a balanced condition.  The slider's position is remembered on a scale marked in units of pressure.

Other resistance-type transducers combine a bellows or a bourdon tube along with a variable resistor, as display in Figure. The bellows will either expand or contract as pressure changes.  That expansion and contraction causes the attached slider to move along with the increasing slidewire, or decreasing the resistance, and by indicating an increase or decrease in pressure.