English physicist who studied the phenomenon of secondary radiation (the effect whereby a substance subjected to X-rays re-emits secondary X-radiation) and X-ray scattering. He was awarded the Nobel Prize for Physics in 1917 for his discovery that X-ray emissions are a form of transverse electromagnetic radiation, like visible light, and monochromatic.
Barkla was born in Widnes, Lancashire, and studied in Liverpool and at Cambridge. He was professor of physics at King's College, London, 1909–13, when he became professor of natural philosophy at Edinburgh University.
In 1903 Barkla published his first paper on secondary radiation. He found that the more massive an atom, the more charged particles it contains, and it is these charged particles that are responsible for the X-ray scattering. Barkla was one of the first to emphasize the importance of the amount of charge in an atom (rather than merely its atomic mass) in determining an element's position in the periodic table.
Between 1904 and 1907 Barkla found that, unlike the low atomic mass elements, the heavy elements produced secondary radiation of a longer wavelength than that of the primary X-ray beam, and that the radiation from the heavier elements is of two characteristic types. Barkla named the two types of characteristic emissions the K-series (for the more penetrating emissions) and the L-series (for the less penetrating emissions). He later predicted that an M-series and a J-series of emissions with different penetrances might exist, and an M-series was subsequently discovered.