# Bimetallic thermostat

> Holding a temperature with a strip that bends

- Date: 1782
- Era: Industrial age
- Domain: [Time & Measurement](https://lastcodex.org/en/domains/measure/)
- Rebuild stage: IV (Science & industry)
- Difficulty: 3/5
- URL: https://lastcodex.org/en/inventions/bimetallic-thermostat/

A bimetallic thermostat holds a temperature steady with a strip made of two metals joined face to face. One metal expands more than the other when warmed, so the strip bends, and its movement opens or closes a damper, a valve or an electric contact. Jean Simon Bonnemain described a heat regulator moved by the unequal expansion of iron and brass in 1782; Andrew Ure named the riveted strip a thermostat around 1830.

![Bimetallic thermostat · Holding a temperature with a strip that bends](https://lastcodex.org/_astro/plate.DvOOWM6e_ZywE9c.webp)

## Legend

1. Brass ruler, which expands more
2. Steel ruler, which expands less
3. Rivets joining the two rulers
4. Fixed end clamped in an iron block
5. Link with adjusting nut, and lever
6. Damper in the warm-air pipe

## What it is

A thermostat keeps a temperature close to a chosen value by letting more or less heat in. In a bimetallic thermostat the part that senses the temperature is a strip of two metals that expand by different amounts, joined along their whole length. It needs no power of its own: the heat bends the strip, and the bending does the work.

In the kind Andrew Ure described in his Dictionary of Arts, Manufactures, and Mines of 1839, a straight bar of brass and steel riveted face to face is fixed at one end. Its free end pulls a lever that turns a damper in a pipe bringing warm air into a room. When the room warms, the bar bends and the damper closes; when it cools, the bar straightens and the damper opens again. Ure listed uses in hothouses, water baths, stills, boilers and the ventilation of rooms.

## How it works

![The same strip of brass riveted onto steel drawn twice, fixed at one end in an iron block: straight when cold, and curved downwards when warmed by a candle beneath, with the brass on the outside of the curve](https://lastcodex.org/_astro/v3.3Wd0CiCY_1h1FFT.webp)
_Warmed, the brass grows more than the steel and has to take the longer, outer side of the curve._

Solids grow slightly when they are warmed, and different materials grow by different amounts: brass more than steel, aluminium more than paper. If two strips are joined so firmly that they cannot slide over each other, the one that grows more has to take the longer, outer side of a curve. The strip bends towards the metal that expands less, and straightens again as it cools. A [thermometer](/en/inventions/thermometer/) uses the expansion of a liquid to give a reading; the strip turns expansion into a movement that can push a lever.

The movement is small and depends on the strip. Stephen Timoshenko, who worked out the theory in 1925, showed that the curvature grows with the difference in expansion between the two metals and with the change of temperature, and that a thinner strip bends more. A longer strip moves its tip further. Ure set spiral bars round a turning register, and Warren Johnson coiled his strip; Ure also used levers to enlarge the movement before it reached the valve or the damper.

What the movement drives depends on the design. In the regulators of Bonnemain and Ure, the bar turns a damper or a cock a little at a time, so the supply of heat follows the temperature. In an electric thermostat the strip opens or closes a contact, which switches a circuit on or off. In every case there is a way to choose the temperature: an adjusting nut on the link, a screw that moves the fixed end, or a contact that can be set nearer or further away. Each of them changes how far the strip must bend before it acts.

## How to do it

![A strip of white paper glued to aluminium foil, held in a wooden clothes peg on a block and curled upwards away from a small foil contact on a second block, above a closed glass jar of water, with wires to a holder for two AA cells, a resistor and an unlit LED](https://lastcodex.org/_astro/v2.CuSbUfkE_1U8BgT.webp)
_Warmed from below, the foil-and-paper strip curls away from its contact and switches the LED off._

A strip of kitchen foil glued to paper bends like a bimetal, because aluminium expands more than paper; science centres use it to show the effect. Cut a strip of printer paper and a strip of foil, each about 1 cm wide and 20 cm long, and glue them together over their whole length with a glue stick, smoothing out any bubbles. Make a second strip of paper glued to paper, as a control. Let both dry flat overnight.

To turn the strip into a thermostat, glue a wooden clothes peg to a block and clamp one end of the foil strip in it, foil side down, so that the strip sticks out level. Glue a small square of foil to a second block under the free end, so that the tip of the strip rests on it. Join a holder for two AA cells, a red LED and a resistor of about 100 ohms in a loop, and break the loop at the strip: one wire is pressed against the foil inside the peg, the other is taped to the foil square. Put the longer leg of the LED towards the positive side. When the strip rests on its contact, the LED lights; here it stands for the heater.

Fill a glass jar with hot water from the tap, screw the lid on tightly so that no steam escapes, dry the outside and stand it under the middle of the strip, a few centimetres below it. As the strip warms, it curls upwards, its tip leaves the contact and the LED goes out: the heater has been switched off. Take the jar away, and as the strip cools it flattens, touches the contact and the LED comes on again. Hold the paper-only strip over the jar in the same way: it barely moves, because both its layers expand alike. Raising or lowering the contact block changes the warmth at which the light goes out, like the adjusting nut of Ure's thermostat.

## History and evolution

Clockmakers met the problem first, because a pendulum rod grows longer in the warm and the clock loses time. Between 1725 and 1728 John Harrison developed a gridiron pendulum, made of alternating rods of two metals arranged so that the expansion of one cancelled that of the other, and fitted it to the precision clocks he built with his brother James. For his third sea clock, H3, begun in 1740, he made a strip of brass and steel joined together, which Royal Museums Greenwich counts among the lasting inventions of that [clock](/en/inventions/mechanical-clock/). In H4, of 1759, the strip that Harrison called his thermometer kirb carries at its free end the pins that hold the balance spring, so that the working length of the spring changes with the temperature. These strips corrected the rate of a clock and controlled no heat.

![A small copper boiler on a brick hearth, cut away to show a lead tube in the water with an iron rod inside, a brass collar at its top lifting a bent lever, a counterweighted beam and a rod down to a swinging flap over the air inlet of the fire, and a small dial on top](https://lastcodex.org/_astro/v1._kCHS7FQ_5r3xy.webp)
_Bonnemain's regulator as it was published: the lead tube grows longer than the iron rod inside it and closes the air inlet of the fire._

The oldest dated heat regulator moved by two metals is French. On 14 August 1782 Jean Simon Bonnemain lodged a sealed memoir on his "régulateur du feu", with drawings, at the Academy of Sciences in Paris. The historian Emmanuelle Gallo, who studied it, describes a double rod of iron and brass whose unequal expansion moved a lever that opened or closed a door letting air into the furnace, and so changed the strength of the fire. Bonnemain used it on the incubators of the chicken farm he ran near Paris, and listed greenhouses, baths and carriage heating among its uses. Otto Mayr, a historian of technology, dates his patent to 1783. The version Bonnemain published in 1824, which Andrew Ure described in 1839, had an iron rod inside a lead tube standing in the water of the boiler, and a dial to set it between strong and weak heat.

Ure, a Scottish chemist who had met Bonnemain in Paris around 1815, patented a regulator made of two thin bars of different metals riveted or soldered face to face. He gives the year as 1831 in his Dictionary; other accounts say 1830. He called it a thermostat. Two long rulers, one of steel and one of hard hammered brass, worked levers that moved valves, cocks, stove registers and ventilators, with the aim, he wrote, of controlling the temperature as well as showing it.

In 1883 Warren S. Johnson, a teacher at the State Normal School in Whitewater, Wisconsin, patented an electric "tele-thermoscope". A coil of spring brass and spring steel tilted a beam whose needle points dipped into cups of mercury, closing a circuit that showed whether the room was too cold or too warm and rang a bell for the attendant. The company he founded became Johnson Controls. Royal Museums Greenwich notes that the bimetallic strip is still used in thermostats of all kinds.

## Key facts

- Oldest dated regulator moved by two metals: Jean Simon Bonnemain, iron and brass, Paris, 1782
- Harrison's bimetallic strip: In his sea clock H3, begun in 1740
- Ure's thermostat: Riveted rulers of steel and hard brass, patented in 1830 or 1831
- Electric thermostat with a bimetal coil: Warren S. Johnson, United States patent, 1883

## Requires

- [Steel](https://lastcodex.org/en/inventions/steel/): Iron with just enough carbon (c. 1,000 BCE)
- [Bronze](https://lastcodex.org/en/inventions/bronze/): Copper and tin, harder together (c. 3,300 BCE)
- [Thermometer and barometer](https://lastcodex.org/en/inventions/thermometer/): Temperature and air pressure read on glass (c. 1654)

## Sources

- [Jean Simon Bonnemain (1743–1830) and the Origins of Hot Water Central Heating (Proceedings of the Second International Congress on Construction History, 2006, pp. 1043–1060)](https://heatinghelp.com/assets/documents/1043-1060.pdf), Emmanuelle Gallo
- [The Origins of Feedback Control (Scientific American, October 1970)](https://archive.org/details/mayro-feedback-en), Otto Mayr, Scientific American (text: Internet Archive)
- [A Dictionary of Arts, Manufactures, and Mines (1839): Heat-regulator; Thermostat](https://archive.org/details/b2930345x), Andrew Ure, Longman (scan: Internet Archive, Wellcome Collection)
- [A Brief History of Thermostats: The Inventors](https://thermostat-recycle.org/a-brief-history-of-thermostats-the-inventors/), Thermostat Recycling Corporation
- [Electric Tele-Thermoscope, US patent 281,884 (24 July 1883)](https://patents.google.com/patent/US281884A/en), Warren S. Johnson (Google Patents)
- [Warren Johnson, the Father of Thermostats and Automated Room Temperature Control](https://ipwatchdog.com/2018/07/16/warren-johnson-father-thermostats-room-temperature-control/), IPWatchdog
- [Marine timekeeper H3](https://www.rmg.co.uk/collections/objects/rmgc-object-79141), Royal Museums Greenwich
- [Longcase clock by James and John Harrison, 1728 (three precision clocks of 1725–1728, gridiron pendulum)](https://collection.sciencemuseumgroup.org.uk/objects/co8557977/longcase-clock-by-james-and-john-harrison-longcase-clock-grasshopper-escapement), Science Museum Group
- [The Principles of Mr. Harrison's Time-Keeper, with Plates of the Same (London, 1767): the thermometer kirb](https://archive.org/details/principlesmrhar00unkngoog), Commissioners of Longitude (scan: Internet Archive, Ghent University Library)
- [Timeline of mechanical engineering innovation: 1725–1728 John Harrison, gridiron pendulum](https://ethw.org/Timeline_of_mechanical_engineering_innovation), ASME / Engineering and Technology History Wiki
- [Analysis of Bi-Metal Thermostats](https://doi.org/10.1364/JOSA.11.000233), S. Timoshenko, Journal of the Optical Society of America 11 (1925)
- [Bimetal strip (activity)](https://www.sciencebuddies.org/stem-activities/bimetal-strip), Science Buddies
- [Curling Metals](https://www.coxsciencecenter.org/sites/default/files/Curling%20Metals.pdf), Cox Science Center and Aquarium
- [Bimetallic strip (lecture demonstration)](https://www.physics.colostate.edu/?p=9440), Colorado State University, Department of Physics

Fact-checked on 2026-10-18.

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The Last Codex · https://lastcodex.org/en/inventions/bimetallic-thermostat/ · © The Last Codex. Texts, plates and figures: CC BY-NC-ND 4.0.