William Thomson, 1st Baron Kelvin, OM, GCVO, PC, PRS, FRSE (26 June 1824 – 17 December 1907) was a British mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy at the University of Glasgow for 53 years, he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging discipline of physics in its modern form. He received the Royal Society's Copley Medal in 1883, was its President 1890–1895, and in 1892 was the first British scientist to be elevated to the House of Lords.
Absolute temperatures are stated in units of kelvin in his honour. While the existence of a lower limit to temperature (absolute zero) was known prior to his work, Kelvin is known for determining its correct value as approximately -273.15 degrees Celsius or -459.67 degrees Fahrenheit. The Joule–Thomson effect is also named for him.
He is buried in Westminster Abbey.