‘The last of the great classical physicists’ – Brian Maye on William Thomson, Lord Kelvin

He proposed an absolute temperatures scale, which is measured in ‘kelvin units’ thanks to his analysis

William Thomson, Baron Kelvin, was born in Belfast on June 26th, 1824

“Lord Kelvin was a towering presence in the world of physics for more than half a century. He played a defining role in its emergence as a distinct subject and has often been called the last of the great classical physicists,” wrote Denis Weaire in the Dictionary of Irish Biography.

Kelvin was Belfast-born William Thomson, the bicentenary of whose birth occurs on June 26th.

He was born at College Square East, the son of James Thomson, a headmaster, and Margaret Gardner.

His father, who was widowed in 1830, tutored him from an early age. The family was of Scottish background and when his father was appointed professor of mathematics at Glasgow University in 1831, the family moved there. By 1834, William had already matriculated at the university, where he and his brother, James, proved prodigious students, and at only 16, he went to Cambridge, where he again excelled. There he won the Smith’s Prize (awarded in maths and theoretical physics for original research), in which one of his examiners is said to have remarked to another: “You and I are just about fit to mend his pens.”


After a year as a Peterhouse College, Cambridge, fellow, which involved time spent in Paris (he’d already been tutored in French in Paris at the end of the 1830s), he was appointed professor of natural philosophy at Glasgow University in 1846 at the age of 22, lecturing where he’d been a first-year student only a few years before.

He occupied that position for 52 years, despite many offers of appointments in more prestigious locations elsewhere. According to Denis Weaire, “he saw positive advantages in remaining remote from the energy-sapping centres of power and bureaucracy that lay to the south”.

Only some of his scientific achievements can be referred to in a piece of this length. He had a highly enquiring mind, as can be seen in his extensive notebooks now in Cambridge University Library, and applied himself to a wide range of research. “Optics, elasticity, electricity, magnetism, thermodynamics, hydrodynamics, navigation, geophysics, crystallography, metrology and telegraphy by no means exhaust the contents of his more than 650 papers,” according to Denis Weaire.

He proposed an absolute temperatures scale, which is measured in “kelvin units” thanks to his analysis. While the coldest possible temperature or absolute zero was known about before him, it was he who calculated its correct value. The “Joule-Thomson effect”, which concerns temperature change of a real gas or liquid in thermodynamics, is additionally named in his honour, and he shares credit for the key concept of entropy in the theory of thermodynamics.

He is also credited with inventions such as air cooling by refrigeration, one of many patents he took out.

His most commercially successful work was in undersea telegraphy. When the first attempt at laying a transatlantic cable failed, his ideas for improvements were accepted by the backers of the scheme. His main contribution was using a mirror galvanometer (it shows it has sensed an electric current by deflecting a light beam with a mirror), which he’d designed himself. This success led to his also contributing to the laying of the French Atlantic cable in 1869 and Brazilian cables in 1883. In solving these and other problems, he displayed his knowledge of maths, his deep understanding of the properties of materials and his practical skills; “his many investigations of materials formed one of the foundation stones of today’s solid-state physics,” according to Denis Weaire, who pointed out that it retains many of the terms he used, such as “permeability”, “susceptibility” and “bulk modus”.

Among his few shortcomings as a physicist that Weaire referred to was his reluctance to accept the electromagnetic theory of light advanced by James Clerk Maxwell, a more serious shortcoming, Weaire believed, than his more often referred to mistaken estimate of the Earth’s age. Thomson estimated that it was 20 million to 40 million years old, whereas it’s now regarded as 4.54 billion, but discoveries after his death greatly helped to improve calculations.

He was elected Fellow of the Royal Society in 1856, knighted in 1866, awarded the Copley Medal in 1883, became Lord Kelvin in 1892 and was president of the Royal Society 1890-1895. In addition, he received many foreign academic awards.

He never denied his Irishness but strongly opposed Home Rule. Widowed in 1879, he married his second wife, Frances Anna, in 1874; there were no children in either marriage. In later years, he lived in his substantial mansion built on the Ayrshire coast and sailed his yacht there. He died on December 17th, 1907, and was buried in Westminster Abbey, near Isaac Newton, with representatives from all over the world attending his funeral.

There is a museum in his memory in Glasgow, various portraits of him in Cambridge and Glasgow, and a statue in Belfast’s Botanic Gardens looks towards his birthplace.