PurdueMET - 

This is the first video in a series showing how strain gauges work. In this video, I show you how to calculate the resistance of a wire and how that resistance rises as the wire is stretched due to a tensile load. I also explain how this allows us to make a strain sensor

as a wire is stretched, its length increases changing the resistivity of the wire.

a typical strain gauge

typically the gauge factor is not much more than 2. it is provided by the strain gauge manufacturers

A steel torsion bar had already been strain gauged before I began my project

PREVIOUSLY PREPARED STRAIN GAUGE