By far, the most famous Professor of Natural Philosophy at the University of Glasgow was William Thomson, later Lord Kelvin, who held the chair from 1846 (when he was 22) to 1897. He was one of the most distinguished scientists of his age and made a name for himself and for Glasgow University in many areas of physics such as thermodynamics, electricity, navigation and geology. His name has been given to the unit of temperature, the Kelvin.

He was born in Belfast on 26 June, 1824. His father became a Professor of Mathematics at Glasgow in 1834. This allowed Kelvin to enter the University at the age of ten. He further studied at Cambridge and Paris before returning to Glasgow for his Professorship, which he got with the help of his father, who was still a Maths Professor.

He started with a run-down one-person department which, in the next half century, he built into a modern department with professors and lecturers teaching students who came, not only from Glasgow, but from all over the world. In the 1890s, these included Japanese students who returned home, trained in modern science, to contribute to the rapid modernisation of their country. Thomson introduced teaching methods which are familiar to students today and combined lectures with laboratory work. These have been widely copied in physics departments throughout the world.

Kelvin first defined the absolute temperature scale in 1847, which is now named after him. Having been asked how long it takes for an electrical signal to travel down a wire, he got involved in submarine cables in 1854. Thus he was involved in the laying of the first transatlantic cable. He also was involved in the evolution debate. Kelvin believed that the sun was heated by the gravitational contraction of the sun (radiation having not been discovered yet). This theory, however, disagreed with evolution, as Kelvin theorised that the sun had been too hot for life, only a million years before.

Kelvin was knighted in 1866 and was created Baron Kelvin (The Kelvin being a small river that flows along the edge of the University grounds) of Largs in 1892. He died in 1907 and was buried in Westminster Abbey in London.

He is still revered in the department he led for so long, now based in the Kelvin Building and a small museum of his apparatus is permanently on display, including his copy (in Latin) of Sir Isaac Newton's  Mathmatica Principia, which includes such scribbles as, 'This is a load of rubbish!'