Origins of the modern thermostat

In 1883, Warren Johnson filed patent 281 884, using a bimetallic strip immersed in mercury to indicate the temperature of a room via a bell and a control panel. This system, the ancestor of the modern thermostat, gave rise to an entire industry of building climate control.

A school's daily challenge

The story begins in the hallways of the Whitewater Normal School, a school in Wisconsin where Warren Johnson has been teaching since 1876. At that time, regulating the heat in a building was not automatic. A human had to do the job, and this role fell to the school's janitor.

Manual temperature regulation

This person literally had to walk through each classroom every hour to check the ambient temperature. If a room was too cold or too hot, he then went down to the basement to manually adjust the boiler registers. A repetitive, time-consuming, and ultimately somewhat absurd task when one thinks about it.

A scientist’s insight

Warren Johnson, however, observes this daily routine from his classroom. As a practical scientist, he says there must be a smarter way to convey this information without needing a full-time person. It was from this frustration, however mundane, that one of the most used objects of our modern daily life would be born.

The bimetallic strip mechanism

The bimetallic strip touching mercury: the mechanical trick behind patent 281 884. Johnson does not just dream of a solution, he conceives it. The result of his work leads to the American patent number 281 884, filed on July 24, 1883, which describes what can be considered the direct ancestor of the electric room thermostat.

How it works

The principle is based on a component still familiar to those interested in antique objects: the bimetallic strip. It consists of two different metals, glued together, which expand at different rates under the effect of heat. This slight difference causes the bimetallic strip to curve, proportional to the room temperature.

Functionality of the device

In Johnson's device, this curvature eventually causes the bimetallic strip to dip into a small amount of mercury, closing an electrical circuit. A bell then rings, and a control panel installed in the basement displays a simple message, "warm" or "cold," depending on the direction of the change.

Impact and legacy

The janitor no longer needs to go up the stairs every hour; he just needs to monitor this panel and adjust the boiler accordingly. Note that this technical description of the mechanism comes from encyclopedic sources and archives related to the company founded by Johnson, with the original text of the patent remaining the most reliable reference for those who wish to verify every detail.

Expanding the innovation

This small school project did not stop there. The National Inventors Hall of Fame today associates Warren Johnson with two major patents, number 281 884 already mentioned, and number 542 733, which complements his work on automated thermal regulation.

Modern climate control industry

From this rudimentary device was born an entire industry, that of building climate control. A century and a half later, the principle remains surprisingly similar, even though mercury has been replaced by electronic sensors.

French regulatory developments

In France, regulation has also followed this logic of automation. Since 2018, installing a new boiler already requires the installation of a thermostat. A decree from 2023 was to go further by imposing room-by-room heating control, a French measure intended to help households better manage their expenses.

Postponement of implementation

This requirement, initially set for January 1, 2027, has however been postponed to 2030 for other buildings, as confirmed by a decree published in the Official Journal on December 27, 2025. Minister of Economy Roland Lescure justified this delay by the desire to give more flexibility to households and professionals to equip themselves.

Current challenges in France

A major issue since, according to Europe 1, 27 million French households remain to be equipped. However, the Ademe estimates up to 15% energy savings allowed by room-by-room control, for an investment of between 60 and 250 euros for purchase, and between 150 and 300 euros for installation.

Legacy of an invention

This reminds us that the small bell invented to ease a janitor in Wisconsin continues, in a transformed form, to be a subject of debate in our administrations more than a century later. What is striking in this story, ultimately, is the distance traveled between a simple constraint, that of a teacher tired of seeing a janitor go up and down the stairs, and an object now central to our current energy policies.