A bimetallic strip is two metals with different coefficients of thermal expansion bonded face-to-face. When temperature changes, one side wants to grow more than the other, but the bond forces them to stay the same length — so the strip bends. Heat one side and it curls toward the low-expansion metal. Cool it and it curls the other way. This is one of the oldest, cheapest, and most reliable ways to turn temperature directly into motion, no electronics required.

The classic pairing is steel + brass or, in higher-precision instruments, Invar (α ≈ 1.2 × 10⁻⁶ /°C) bonded to a high-expansion nickel alloy (α ≈ 20 × 10⁻⁶ /°C). Invar's near-zero expansion coefficient gives you a large differential, which means large deflection per degree.

Where you'll find them:

• Mechanical thermostats — a coiled bimetallic strip rotates as it heats, tilting a mercury switch or opening a contact. Your grandparent's round Honeywell wall thermostat is exactly this.
• Thermal-magnetic circuit breakers — the thermal element is a bimetallic strip that bends and trips the mechanism under sustained overcurrent. The magnetic element handles short-circuit spikes; the bimetal handles the slow-burn overloads.
• Automotive chokes, coffee maker thermostats, iron temperature controls, toaster timers, fluorescent tube starters — all classic bimetal applications.
• Blinker relays — old-school "click-click" turn signals used a bimetal strip that heated up, bent open a contact, cooled, and closed again. That's where the sound came from.

Rule of thumb — curvature of a bimetallic strip:

1/R ≈ 6 · (α₂ − α₁) · ΔT / t

Where R is the radius of curvature, ΔT is the temperature change, and t is the total strip thickness (assuming equal-thickness layers). Two takeaways: thinner strips bend more per degree, and deflection scales linearly with the expansion coefficient difference. This is why manufacturers stack many turns into a spiral coil — small radial deflections multiply into large angular rotations at the tip.

Failure modes worth knowing: repeated cycling causes fatigue at the bond interface (delamination); overheating past the alloy's stress-relief temperature causes permanent set — the strip no longer returns to its zero position, and calibration drifts. This is why old thermostats read low and old breakers trip early.