A strain gage is a small section of very fine wire that changes resistance when its dimensions are changed. The fine wire grid is bonded to thin plastic film, and two leads (or two terminals) are available to connect the gage to measuring instruments.

In use, a strain gage is attached to the surface of the object to be measured with an appropriate adhesive. Consider two strain gages that are glued to the top and bottom of a metal cantilever beam. When a force is applied to the beam, it bends down in proportion to the force (Hooke’s Law). As the bar bends downward, the top of the bar stretches as it is placed in tension. The top strain gage is also stretched, and its resistance increases slightly because the length of wire in the gage increases and the width of the wire decreases. For the strain gage on the bottom of the bar, all of these effects are reversed. The bottom side of the beam is placed in compression and the gage decreases in resistance. These changes in resistance are small, but they can be measured accurately with the proper circuit.

In this activity, you will create a Wheatstone half-bridge circuit that uses two strain gages, and measure the strain on a cantilever beam as a load is applied at the end of the beam.