The Kaufmann–Bucherer–Neumann experiments measured the dependence of the inertial mass (or momentum) of an object on its velocity. The historical importance of this series of experiments performed by various physicists between 1901 and 1915 is due to the results being used to test the predictions of special relativity. The developing precision and data analysis of these experiments and the resulting influence on theoretical physics during those years is still a topic of active historical discussion, since the early experimental results at first contradicted Einstein's then newly published theory (1905), but later versions of this experiment confirmed it. For modern experiments of that kind, see Tests of relativistic energy and momentum, for general information see Tests of special relativity. Historical context In 1896, Henri Becquerel discovered radioactive decay in a variety of chemical elements. Subsequently, the beta radiation from these decays was discovered to be the emission of a negatively charged particle. Later these particles were identified with the electron, discovered in cathode ray experiments by J. J. Thomson in 1897. This was connected with the theoretical prediction of the electromagnetic mass by J. J. Thomson in 1881, who showed that the electromagnetic energy contributes to the mass of a moving charged body. Thomson (1893) and George Frederick Charles Searle (1897) also calculated that this mass depends on velocity, and that it becomes infinitely great when the body moves at the speed of light with respect to the luminiferous aether. Also Hendrik Antoon Lorentz (1899, 1900) assumed such a velocity dependence as a consequence of his theory of electrons. At this time, the electromagnetic mass was separated into "transverse" and "longitudinal" mass, and was sometimes denoted as "apparent mass", while the invariant Newtonian mass was denoted as "real mass". On the other hand, it was the belief of the German theoretician Max Abraham that all mass would ultimately prove to be of electromagnetic origin, and that Newtonian mechanics would become subsumed into the laws of electrodynamics. In 1904, Lorentz proposed that the mass along the trajectory of motion (longitudinal mass) and the mass in the directions perpendicular to the motion (transverse) are different. In his theory, the longitudinal mass and the transverse mass , where is the Lorentz factor and is the rest mass of the electron. The concept of (transverse) electromagnetic mass , which was based on specific models of the electron, was later transmuted into the purely kinematical concept