Paul Ulrich Villard (28.09.1860-13.I 1934) was a French physicist and a member of the Paris Academy of Sciences (1908). He was born in Lyon. He graduated from the École Normale. He was a professor at a college in Paris.
His work concerned the study of radioactivity and physical chemistry. He studied hydrates, and in particular discovered a hydrate of silver; he studied radioactive radiation and invented several radiological instruments. In 1899 he discovered the formation, under the action of light, of a reversed photographic image (the Villard effect), and in 1900 gamma rays.