The Kelvin scale is the temperature scale used in the International System of Units (SI). The SI is exactly what it says on the tin: a standardized measurement system to be used by everyone.
A Note on Notation
The Kelvin scale, as defined in the SI, does not use degrees. The units are kelvins, not “degrees Kelvin.” As for capitalization, the K is uppercase when referring to the scale, but lowercase for the unit. Of course, this only matters for pedantry.
The History of the Kelvin Scale
The history of the Kelvin scale begins in 1848 with British physicist and engineer William Thomson, First Baron Kelvin. At this time, Thomson calculated absolute zero as being approximately −273°C, which we now know is accurate.
Absolute zero (0 K, or −273.15°C) is the point at which all thermal motion of particles theoretically ceases. Nothing can be cooled below it, making it a natural starting point for a temperature scale. This is what makes Kelvin different from Celsius and Fahrenheit, which both have arbitrary zero points. Celsius sets zero at the freezing point of water, and Fahrenheit sets it at the temperature of a specific brine solution. Kelvin starts at the lowest temperature physically possible, which is why it is the preferred scale in physics and chemistry. A temperature in kelvins is always positive, which simplifies equations in thermodynamics and statistical mechanics where negative temperatures would cause problems.
Further reading:
- The BIPM’s official definition of the kelvin in the SI (Bureau International des Poids et Mesures)
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