strain gauge
A strain gauge measures how much something stretches or squashes by turning that deformation into a change in resistance. The trick is delightfully simple: glue a long, thin zig-zag of fine wire (or etched metal foil) firmly onto the surface you care about. When the surface stretches, the wire is stretched too — it gets a little longer and thinner — and since a longer, thinner wire has more resistance, the gauge's resistance creeps up by a tiny fraction. Stretch a beam, and the gauge bonded to its top reads a resistance rise; the bottom, being compressed, reads a fall. This is how a bathroom scale, a crane's load limit, and a wind-tunnel model all sense force.
The catch is that the change is minuscule — a typical 1000-microstrain stretch (0.1%) shifts a 350 Ω gauge by under half an ohm — far too small to read directly against drift and temperature. So strain gauges are almost always wired into a Wheatstone bridge, which cancels the huge fixed resistance and leaves only the tiny imbalance, then an instrumentation amplifier multiplies that up. The sensitivity is captured by the gauge factor G (≈2 for metal foil), defined as the fractional resistance change per unit strain. Cleverly placing four gauges — two stretching, two compressing — in a full bridge both quadruples the signal and automatically cancels temperature drift, which is the heart of every load cell.
Temperature is the strain gauge's nemesis: thermal expansion alone can dwarf the real signal, which is why a single naked gauge is nearly useless. The full Wheatstone bridge's beauty is that heat affects all four gauges equally, so the bridge stays balanced — the deformation, which affects them differently, is all that survives.