The History of the Thermometer: From Galileo to the Boltzmann Constant
Source: NIST. Published: 2025-09-15.
Galileo's Thermoscope
The first device to measure temperature variation — the thermoscope — is generally attributed to Galileo Galilei, who described it around 1593. The device was a glass tube with a small sphere at one end, filled with water and dipped into a vessel of liquid; as temperature changed, the water level in the tube rose or fell. The thermoscope could detect temperature change but had no numerical scale and was sensitive to atmospheric pressure as well as temperature, making its readings ambiguous. Over the following century, various investigators improved the design: they enclosed the liquid to exclude atmospheric pressure effects, replaced water with wine or alcohol (which has a lower freezing point), and began adding numerical scales — though these were arbitrary and different instruments could not be directly compared.
Fahrenheit and the First Reliable Thermometer
Daniel Gabriel Fahrenheit, a Dutch-German instrument maker, produced the first reliable, reproducible mercury thermometers in the early 1700s. Mercury had several advantages over alcohol: it remained liquid over a wider temperature range, had a uniform thermal expansion, and did not wet glass, making the meniscus easy to read. Fahrenheit chose a zero point based on the coldest temperature he could achieve with a mixture of ice, water, and ammonium chloride salt, and set body temperature (later revised to 98.6°F) as another reference point. His thermometers agreed with each other — a crucial innovation — and were quickly adopted by scientists across Europe.
Celsius and the Centigrade Scale
In 1742, Swedish astronomer Anders Celsius proposed a scale based on the two most reproducible physical references available: the freezing point and boiling point of water at standard pressure. In Celsius's original design, 0° was the boiling point and 100° was the freezing point — the scale ran in the opposite direction from what we use today. His colleague Carolus Linnaeus (or possibly Martin Strömer) reversed the scale to the familiar orientation where 0° is freezing and 100° is boiling, and this 'centigrade' scale was gradually adopted across Europe and eventually the world. The scale was formally named Celsius in 1948 to honour its originator and to distinguish it from other centigrade scales.
Absolute Zero and Lord Kelvin
By the mid-nineteenth century, physicists had recognised that there must be a lowest possible temperature — a point at which molecular motion ceases. William Thomson (later Lord Kelvin) proposed in 1848 that this absolute zero be used as the starting point for a thermodynamic temperature scale. On the Kelvin scale, absolute zero is 0 K; the freezing point of water is 273.15 K; the boiling point is 373.15 K. The Kelvin scale uses the same degree size as Celsius — the two differ only in their zero point. The Kelvin became the SI base unit of temperature in 1960, with the zero point fixed at absolute zero and the scale calibrated so that the triple point of water (where ice, liquid water, and water vapour coexist) was exactly 273.16 K.
The 2019 Redefinition: Temperature from the Boltzmann Constant
Before 2019, the kelvin was defined by fixing the temperature of the triple point of water at exactly 273.16 K. This tied the unit to a specific material property — a physical phenomenon that, while highly reproducible, is subtly sensitive to the isotopic composition of the water used. The 2019 SI revision redefined the kelvin by fixing the value of the Boltzmann constant k to exactly 1.380 649 × 10⁻²³ joules per kelvin. Because the joule and kelvin are now both defined through fixed fundamental constants, the kelvin is fully independent of any material property. Any measurement of temperature is now, in principle, a measurement of molecular kinetic energy expressed in units of the Boltzmann constant — a conceptually cleaner foundation than any fixed-point reference.
Modern Temperature Measurement and the Converter
Today temperature is measured using platinum resistance thermometers calibrated against the ITS-90 scale, thermocouples, radiation thermometers, and increasingly, primary acoustic gas thermometers that measure thermodynamic temperature directly. The Unit Master Temperature Converter supports Celsius, Fahrenheit, and Kelvin — the three scales encountered in everyday life and scientific work. The conversion formulas are exact: K = °C + 273.15; °F = (°C × 9/5) + 32. The exact values reflect the defined relationships among the three scales, not approximations — a consequence of the careful history described in this article, through which the major temperature scales were precisely related to each other and ultimately to the fundamental constant of physics that defines heat itself.
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