The four arms of the unstressed strain gage are equal in length and thickness. Under these conditions they have the same electrical resistance.

When the strain gage is distorted under a torsional load two of the arms are stretched longer (and thinner) and two become shorter (and wider). The electrical resistance has increased in the longer arms and decreased in the shorter arms. These resistance changes are proportional to the torque on the shaft and can be accurately measured with a Wheatstone bridge circuit.