Jan Kazimierz Danysz (11 March 1884 – 4 November 1914) was a French physicist of Polish extraction. He was an assistant of Maria Skà Âodowska-Curie and notable in the development of beta spectrometry.
Danysz made considerable advances on the magnetic deflection techniques of , Otto Hahn and Lise Meitner, placing the source (he used radium) in a capillary tube under a slit, with a photographic plate in the same horizontal plane. By this means the known number of lines (later understood to be conversion lines) superimposed on the beta energy spectrum of radium B (RaB) + radium C (RaC) went from 9 to 27 lines (later work by Harold Roper Robinson and Ernest Rutherford found 64 lines; 16 lines from RaB and 48 lines from RaC). He finished his doctoral thesis in 1913, and by 1914 he was considered by Rutherford as a leading researcher into beta decay, but he did no further work. He enlisted in the French army in 1914 and was killed in action near Cormicy during World War I.