a strain gauge
/ strayn gayj /
A strain gauge feels how much something stretches. Glue a thin metal foil to a beam, then bend the beam: the foil stretches with it, and a stretched wire gets slightly longer and thinner, which makes its resistance rise a tiny bit. Squeeze the beam and the foil shortens and its resistance falls. By reading that minute resistance change you are, in effect, watching the material flex — a way to turn force, weight, pressure, or torque into electricity.
The gauge is a long, hair-thin conductor folded into a tight zig-zag so a lot of its length lies along the direction you want to measure. Its resistance change per unit stretch is set by the gauge factor, usually about 2 for metal-foil gauges. The brutal truth is how small the signal is: a typical strain might be 0.001 (0.1 percent stretch), which with a gauge factor of 2 changes a 350 Ω gauge by only 2 times 0.001 times 350 = 0.7 Ω — a fifth of a percent. You cannot read that reliably as an absolute resistance, so a strain gauge almost always lives in a Wheatstone bridge, which turns the tiny resistance change into a small differential voltage that an instrumentation amplifier can boost. Putting gauges in multiple arms of the bridge multiplies the signal and cancels temperature drift.
Why this matters: strain gauges are the heart of nearly all electronic weighing and force measurement — kitchen and truck scales, crane safety limits, engine test rigs, structural monitoring of bridges and aircraft. The honest caveats: the signal is so faint that bridge balance, amplifier offset, and temperature effects all matter enormously; the gauge must be bonded perfectly and the assembly calibrated against known loads; and because metal also changes resistance with temperature, you rely on the bridge arrangement to subtract that drift out. It is precision sensing built on a change you can barely measure.
A 350 Ω strain gauge with a gauge factor of 2 experiences a strain of 0.0005 (500 microstrain). Its resistance changes by only 2 times 0.0005 times 350 = 0.35 Ω. Alone that is hopeless to read, but in a Wheatstone bridge on 5 V it produces about 1.25 mV of differential signal — small, but exactly what an instrumentation amplifier is made to handle.
Stretch changes resistance a hair; a bridge makes that hair measurable.
The resistance change is a fraction of a percent, so a strain gauge is essentially never read directly — it needs a Wheatstone bridge plus high-gain, low-offset amplification, and temperature compensation built into the bridge arrangement.