Strain Gauge:

Since the wire grid is distorted through elastic deformation, its length is raise, and its cross-sectional area decreases.  Those changes cause an increase in the resistance of the wire of the strain gauge.

Figure: Strain Gauge

That change in resistance is used as the variable resistance in a bridge circuit which gives an electrical signal for indication of pressure.  Figure describes a strain gauge pressure transducer.

Figure: Strain Gauge Pressure Transducer

An increase in pressure at the inlet of the bellows causes the bellows to expand.  An expansion of the bellows moves a flexible beam to that a strain gauge has been attached.  A movement of the beam causes the resistance of the strain gauge to modify.  A temperature compensating gauge compensates for the heat generated through current flowing by the fine wire of the strain gauge.