Two Metals, One Simple Idea
Every winter morning, before you've even opened your eyes, a small device on your wall has already made a quiet decision on your behalf. It sensed the air growing cold, completed an electrical circuit, and summoned your furnace to life — all without a microprocessor, battery, or internet connection. The secret behind most traditional thermostats is one of the cleverest tricks in all of engineering: a bimetallic coil.
Imagine a relay race where one runner speeds up in the heat and another stays slow. If you strapped them together at the hip, the faster runner would drag the team into a curve. That's essentially what a bimetallic strip does — except instead of runners, it uses two different metals bonded together along their entire length.
The two metals most commonly used are brass and steel (or sometimes invar, a nickel-iron alloy chosen specifically because it barely expands at all). Every metal expands when heated and contracts when cooled, but different metals do so at different rates — a property called the coefficient of thermal expansion. Brass expands roughly twice as much as steel for the same temperature change. When you bond them together in a thin strip, the brass "wants" to grow longer as the room warms up, but the steel resists. The only place all that mechanical tension can go is sideways — so the strip bends, curving toward the steel side.
From Bend to Click
In a traditional thermostat, this strip is wound into a tight coil, which amplifies the tiny bending motion into a more useful rotational movement. As the room cools, the coil tightens and rotates in one direction; as it warms, the coil unwinds in the other. Attached to one end of this coil is a small glass capsule containing liquid mercury — though modern versions use a snap-action electrical contact instead.
When the coil rotates far enough in the cool direction, the mercury (or contact) tips to complete an electrical circuit. That circuit sends a low-voltage signal — typically 24 volts — to a relay switch in your furnace or boiler, which in turn switches on the full power needed to fire the burner. When the room warms back up, the coil rotates the other way, the circuit opens, and the furnace shuts off.
No computer needed. No software update required. The metal itself is the logic.
The Role of the Set Point
The dial or slider on your thermostat physically repositions the coil relative to the contact point. Turn the dial to 72°F, and you're mechanically adjusting how far the coil must rotate — how cold it must get — before the circuit closes. You're not programming a computer; you're setting a mechanical threshold, like adjusting the tension on a mousetrap.
Why It Still Matters
Digital and smart thermostats have largely replaced bimetallic coils in new homes, using thermistors — resistors whose electrical resistance changes predictably with temperature — to sense heat far more precisely. But bimetallic strips haven't disappeared. They live inside:
- Circuit breakers (tripping when too much current heats the strip)
- Automotive choke valves
- Pop-up turkey timers
- Countless industrial temperature controls
The physics is too elegant and too reliable to retire.
A Surprising Origin at Sea
Here's the part most people never know: the bimetallic strip was first described and put to practical use by the clockmaker John Harrison in the 1750s — not to control temperature, but to compensate for it. His marine chronometers used a bimetallic strip in the clock mechanism itself, so that as the metal parts expanded and contracted with ocean temperature changes, the strip automatically corrected the clock's rate. He wasn't measuring temperature; he was canceling it out. The same tool that keeps your house warm on a February morning was first invented to keep time accurate at sea.