Rayleigh, Robert John Strutt, 4th Baron Rayleigh (1875–1947)

British physicist. He carried out important investigations in spectroscopy, but is best known for his measurements of the amounts of radioactive materials present in the earth's crust. These suggested that the heat generated by radioactive changes is sufficient to account for the observed flow of heat from the interior to the surface of the earth, and that, contrary to earlier beliefs, no appreciable cooling of the surface is likely in the course of many millions of years. He also showed that, by measuring the extent to which the end products of radioactive changes had accumulated in rocks since they were laid down, their ages could be estimated.

Rayleigh was born in Terling, Essex, England. He was educated at Eton and Trinity College, Cambridge. He was elected a Fellow of the Royal Society in 1905, and received the Rumford Medal in 1920. From 1908 he was professor of physics at Imperial College of Science and Technology, but resigned in 1920 when he became 4th Baron Rayleigh. He wrote a life of his father, fellow scientist John Rayleigh, in 1924, and of the English physicist J J Thomson in 1942.