of relativistic mass which concerns all forms of energy, not only electromagnetic energy. Currently, however, the concept of relativistic mass, although still frequently referred to in popular works on relativity, is now seldom used among professional working physicists, and has been superseded by the expressions for relativistic energy and momentum, which also predict that the speed of light cannot be reached by massive bodies. This is because all of those relations involve the Lorentz factor: Therefore, the Bucherer–Kaufmann–Neumann experiments can be seen as early tests of relativistic energy and momentum. (For the following historical description of the experiments, the concepts of "transverse" or "relativistic mass" are still used). Kaufmann's experiments First experiments Walter Kaufmann began to experiment with beta rays using a device similar to a cathode ray tube, where the source of the electrons was the decay of radium that was placed in an evacuated container. (See Fig. 1) Such rays emitted from radium were called "Becquerel rays" at that time. Contrary to the then known cathode rays which reached speeds only up to 0.3c, c being the speed of light, Becquerel rays reached velocities up to 0.9c. However, since beta particles have different velocities, the radiation was inhomogeneous. Therefore, Kaufmann applied electric and magnetic fields aligned parallel to each other, so that the deflections caused by them were perpendicular to each other. Their impacts on a photographic plate produced a deflection curve, whose individual points corresponded to a certain velocity and a certain mass of the electrons. By reversing the charge on the condenser, thus inverting the electric field, two symmetric curves could be obtained, whose center line determined the direction of the magnetic deflection. Kaufmann published a first analysis of his data in 1901 – he actually was able to measure a decrease of the charge-to-mass ratio, thus demonstrating that mass or momentum increases with velocity. Using Searle's formula (1897) for the electromagnetic energy increase of charged bodies with velocity, he calculated the increase of the electron's electromagnetic mass as a function of velocity: , Kaufmann noticed that the observed increase cannot be explained by this formula, so he separated the measured total mass into a mechanical (true) mass and an electromagnetic (apparent) mass, the mechanical mass being considerably greater than the electromagnetic one. However, he made two mistakes: As shown by Max Abraham, Kaufmann overlooked that Searle's formula only applies in the longitudinal direction, but for