Born as son of a professor of mathematics in Belfast, he started at Glasgow University at the age of 10 and finished at Cambridge in Mathematics and Physical Science. At 22 he became professor of Natural Philosophy in Glasgow, a post he would hold until 1899. In1856 he became a director of the Atlantic Telegraph Company which finished the Atlantic Cable in 1866; a accomplishment for which he was knighted (Baron Kelvin of Largs) the same year. At the same time James Joule was researching the relationship between heat, electricity and mechanical work. Thomson reviewed Joule's paper on the mechanical equivalent of heat which resulted in a seven year co-operation in which they unraveled the mutual relations between heat and work. This was the foundation for modern thermodynamics.

English physicist, initially interested in the efficiency of electric motors, discovered what is now known as Joule's law Q=i²Rt (Q=Heat produced equals the I=current squared times R=Resistance). Motivated by his theological beliefs he began attempting to demonstrate the unity of forces in nature. Although he did not formally claim the formulation of the Law of Conservation of Energy, his experiments were fundamental in bringing that formulation about.

The Joule-Thomson effect is named after James Prescott Joule and William Thomson (Lord Kelvin).

It is based on the fact that non-ideal gasses exhibit intermolecular interactions (cohesion). When these gasses are expanded, the interactions end which will generally cost energy. This energy is drained from the environment causing it to cool down.

The effect is commonly observed in spray cans (deodorant, paint) but is also used in refrigerators and air-conditioners and in the liquefaction of gasses.

In cryocoolers gas at very high pressure in a suitable bottle is allowed to escape through a very small nozzle into a larger volume, which also contains the array-element to be cooled. The time to reach cryogenic temperatures is extremely short (< 1 sec), which makes it well suited for missile applications.