Lord Kelvin located absolute zero with a footnote calculation
In 1848 William Thomson, later Lord Kelvin, published a temperature scale that did not work out. Tucked into a footnote, though, he calculated the coldest possible temperature at minus 273 degrees Celsius, strikingly close to today's accepted minus 273.15. That absolute zero became the starting point of the kelvin.
Earlier scales like Fahrenheit and Celsius, both 18th-century inventions, predated the physics that explains heat. They were anchored to convenient, reproducible points within everyday experience, such as the melting and boiling of water, and for two centuries a Celsius degree meant one hundredth of the gap between them. Those reference points had no deep significance in thermal physics.
Gas experiments pointed to something more fundamental. Between 1787 and 1802, Jacques Charles, John Dalton and Joseph Louis Gay-Lussac found that at constant pressure a gas changes volume in a straight line with temperature, by roughly 1/273 of its volume per degree. Extend that line far enough and a gas at about minus 273 degrees would shrink to nothing. Thomson got his figure by taking the negative reciprocal of 0.00366, the expansion coefficient of an ideal gas.
Thomson wanted a scale that did not depend on any particular substance, based on the idea that heat falling through one degree should yield the same mechanical work at any temperature. That same year James Prescott Joule proposed a different formula for a key function in Thomson's theory. Thomson was doubtful at first, trusting the experimental data gathered by Regnault, but after joint experiments with Joule he accepted by 1854 that Joule had been right.
The modern kelvin keeps Celsius-sized steps but starts at absolute zero, so adding 273.15 converts Celsius to kelvin. In the early 20th century it was often called absolute Celsius. It formally joined the SI in 1954, which set the triple point of water at 273.16 kelvin, and Celsius, Fahrenheit and Rankine were then redefined in its terms. Since the 2019 SI revision the kelvin has been tied to energy via a fixed Boltzmann constant: each kelvin corresponds to exactly 1.380649 times ten to the minus 23 joules of thermal energy.
Source: Kelvin
