respectively, in order to obtain the charge-to-mass ratio as if the electrons were at rest. Since the ratio doesn't vary for resting electrons, the data points should be on a single horizontal line (see Fig. 6). However, this was approximately only in the case when the data were calculated with Lorentz's formula, while the results of Abraham's formula sharply deviated (the red and blue lines represent the average value according to both formulas). The agreement with the Lorentz–Einstein formula was interpreted by Bucherer as the confirmation of the relativity principle and the Lorentz–Einstein theory – a result immediately applauded by Lorentz, Einstein, and Hermann Minkowski. In addition, Bucherer's apparatus was improved in 1909 by his student Kurt Wolz, who also obtained agreement with the Lorentz–Einstein formula (though he didn't compare Abraham's formula with his data, Fig. 7). Even though many physicists accepted Bucherer's result, there still remained some doubts. For instance, Bestelmeyer published a paper in which he cast doubt on the validity of Bucherer's result. He argued that one experiment alone cannot establish the correctness of an important physical law, that Bucherer's result might be significantly distorted by non-compensated rays reaching the photographic plate, and that extensive data protocols and error analysis are necessary. A polemic dispute between those two scholars followed in a series of publications, in which Bestelmeyer argued that Wolz's experiments are affected by the same problems. Hupka Unlike Kaufmann and Bucherer, Karl Erich Hupka (1909) used cathode rays at 0.5c for his measurements. The radiation (generated at a copper cathode) was strongly accelerated by the field between cathode and anode in a highly evacuated discharge tube. The anode serving as a diaphragm was passed by the ray with constant velocity and drew the shadow image of two Wollaston wires on a phosphorescent screen behind a second diaphragm. If a current was generated behind this diaphragm, then the ray was deflected and the shadow image was displaced. The results were in agreement with Lorentz–Einstein, even though Hupka remarked that this experiment didn't represent a definitive result. Subsequently, W. Heil published some papers dealing with criticisms and interpretations of the result, to which Hupka replied. Neumann and Guye/Lavanchy In 1914, Günther Neumann conducted new measurements using Bucherer's equipment, in particular making some improvements to address Bestelmeyer's criticisms, especially the question of non-compensated rays, and making extensive refinements to the data protocols. The calculation method was the same