Applying the discovery that cathode rays pass through thin leaves of metal, Lenard constructed (1898) a cathode-ray tube with an aluminum window through which the rays could pass into the open air. Using a phosphorescent screen, he showed that the rays decreased in number as the screen was drawn away from the tube and that they ceased at a distance. The experiments also demonstrated that the power of substances to absorb the rays depends on their density and not on their chemical nature and that absorption decreases with increasing velocity of the rays. In similar experiments in 1899 he proved that cathode rays are created when light strikes metal surfaces; this phenomenon later became known as the photoelectric effect.
Lenard's extensive research also included studies of ultraviolet light, the electrical conductivity of flames, and phosphorescence. He wrote a considerable number of books on cathode rays, relativity, and related subjects, including Uber Kathodenstrahlen (1906; "On Cathode Rays") and Deutsche Physik, 4 vol. (1936-37; "German Physics").
An ardent supporter of Nazism, Lenard publicly denounced "Jewish"
science, including Albert Einstein's theory of relativity.
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