Celsius ↔ Kelvin Lab Thermometer

Anders Celsius (1701-1744), a Swedish astronomer at Uppsala, proposed in 1742 a temperature scale based on water's phase transitions. In his original publication, Celsius defined 0° as the BOILING point and 100° as the FREEZING point — the reverse of what we use today. Carl Linnaeus, his colleague, flipped the scale to its modern form in 1745, the year after Celsius died of tuberculosis.

A century later, William Thomson (1824-1907, later Lord Kelvin) was wrestling with the implications of Sadi Carnot's 1824 paper on heat engines. Carnot showed that the efficiency of an ideal heat engine depends only on the ratio of source and sink temperatures — but what is "temperature" if not relative to some arbitrary reference like ice? Thomson proposed in 1848 an ABSOLUTE scale where zero is the temperature at which all classical thermal motion ceases. He calculated this point at -273.15 °C using Joseph Gay-Lussac's 1802 finding that gases lose 1/273 of their volume per degree of cooling at constant pressure.

The 1854 paper formalized the absolute scale as the Kelvin scale. For 165 years the kelvin was defined by various physical anchors: until 1948 by the boiling point of water at 1 atm; from 1948 to 2019 by fixing the triple point of water at exactly 273.16 K. The triple-point definition allowed precision to a few parts in 10⁶, but it depended on the isotopic composition of the water — and standard isotopes vary by source.

On May 20, 2019, the General Conference on Weights and Measures retired the triple-point definition. The kelvin is now defined by fixing the Boltzmann constant at exactly k = 1.380649 × 10⁻²³ joules per kelvin. This makes the kelvin a derived consequence of the joule and Boltzmann's relation E = kT. National Metrology Institutes now realize the kelvin using acoustic gas thermometry, dielectric-constant gas thermometry, or Johnson-noise thermometry.

Modern cryogenics has pushed the lower bounds remarkably. Liquid helium-4 at 1 atm boils at 4.15 K; pumping the vapor away reduces it to ~1.5 K. Helium-4 transitions to a SUPERFLUID at 2.17 K (the lambda point). Helium-3 reaches superfluidity at ~2.5 mK. Dilution refrigerators routinely reach 10 mK. Adiabatic demagnetization of nuclear spins has reached 100 picokelvin (10⁻¹⁰ K) — the coldest temperatures ever recorded, in basement physics labs.

At the other extreme, the solar core sits at 1.5 × 10⁷ K (15 million K). Fusion reactors like ITER and JET aim for 1.5 × 10⁸ K (150 million K) — ten times the solar core — because they lack the sun's gravitational confinement and need higher temperatures to overcome the Coulomb barrier between hydrogen nuclei. JET achieved 150 M K sustained for 5 seconds in 2022. SPARC and ITER are projected to reach burning plasma in the 2030s. At LHC heavy-ion collisions, quark-gluon plasma reaches 5.5 × 10¹² K — the hottest temperature ever measured in human experiments.

Every conversion between Celsius and Kelvin in this tool uses the exact offset 273.15 — no approximation. Both scales share the same degree size, so only the zero point differs. The kelvin's status as an absolute scale makes it the natural unit for radiation laws (Stefan-Boltzmann T⁴, Wien displacement, Planck blackbody), gas laws (PV = nRT), and thermodynamics generally — every physics formula involving temperature in a power or exponent requires kelvin.