Alfred Heinrich Bucherer (* 9 July 1863 in Cologne; † 16 April 1927 in Bonn) was a German physicist, who is known for his experiments on relativistic mass. He also was the first who used the phrase "theory of relativity" for Einstein's theory of special relativity. Education He studied from 1884 until 1899 at the University of Hannover, the Johns Hopkins University, the University of Strassburg, the University of Leipzig, and the University of Bonn. In Bonn he habilitated in 1899 and taught there until 1923. In 1903 Bucherer published the first German-language book to be completely based on vector calculus. Theory of relativity Like Henri Poincaré (1895, 1900), Bucherer (1903b) believed in the validity of the Principle of relativity, i.e. that all descriptions of electrodynamic effects should only contain the relative motion of bodies, not of the aether. However, he went a step further and even assumed the physical non-existence of the aether. Based on those ideas he developed a theory in 1906, which also included the assumption that the geometry of space is riemannian. But the theory was vaguely formulated and in 1908 Walther Ritz showed that Bucherer's theory leads to wrong conclusions with respect to electrodynamics. And contrary to Albert Einstein, he didn't connect his rejection of the aether with the relativity of space and time. In 1904 he developed a theory of electrons in which the electrons contract in the line of motion and expand perpendicular to it. Independently of him Paul Langevin developed a very similar model in 1905. The Bucherer-Langevin model was an alternative to the electron models of: Hendrik Lorentz (1899), Henri Poincaré (1905, 1906) and Albert Einstein (1905). in which the electrons are subjected to length contraction without expansion in the other direction and the model of Max Abraham, in which the electron is rigid. All three models predicted an increase of the electron mass if their velocities are approaching the speed of light. The Bucherer-Langevin model was quickly abandoned, so some experimentalists tried to distinguish between Abraham's theory and the Lorentz-Einstein theory by experiment. This was done by Walter Kaufmann (1901–1905) who believed that his experiments confirmed Abraham's theory, and disproved the Lorentz-Einstein theory. But in 1908 Bucherer conducted some experiments as well, and obtained results which seem to confirm the Lorentz-Einstein theory and the principle of relativity. With exceptions like Adolf Bestelmeyer with whom Bucherer had a polemical dispute, Bucherer's