deflection measurements the formula for the transverse direction was important. Therefore, Abraham introduced the "transverse electromagnetic mass" with the following velocity dependence: Kaufmann also made a calculation mistake in deriving the deflection curves. Those errors were corrected by him in 1902. In 1902 and 1903 Kaufmann performed another series of tests with updated and improved experimental techniques. The results were interpreted by him as a confirmation of Abraham's theory and of the assumption that the electron's mass is completely of electromagnetic origin. Hermann Starke conducted similar measurements in 1903, although he used cathode rays limited to 0.3c. The results that he obtained were interpreted by him as being in agreement with those of Kaufmann. Competing theories In 1902, Max Abraham published a theory based on the assumption that the electron was a rigid, perfect sphere, with its charge being distributed evenly on its surface. As explained above, he introduced the so-called "transverse electromagnetic mass" besides the "longitudinal electromagnetic mass", and argued that the entire electron mass is of electromagnetic origin. Meanwhile, Lorentz (1899, 1904) extended his theory of electrons, assuming that an electron's charge was spread throughout its volume, and that in Kaufmann's experiment, its shape would be compressed in the direction of motion and would stay unchanged in the transverse directions. To Kaufmann's surprise, Lorentz could show that his model agreed with his experimental data as well. This model was further elaborated and perfected by Henri Poincaré (1905), so that Lorentz's theory was now in agreement with the principle of relativity. A similar theory was developed by Alfred Bucherer and Paul Langevin in 1904, with the difference that the total volume occupied by the deformed electron was assumed unchanged. It turned out that this theory's prediction was closer to Abraham's theory than to Lorentz's. Finally, Albert Einstein's theory of special relativity (1905) predicted the change of the point-like electron's mass due to the properties of the transformation between the rest-frame of the particle and the laboratory frame in which the measurements were performed. Mathematically, this calculation predicts the same dependence between velocity and mass as Lorentz's theory, although it assumes very different physical concepts. As regards the increase of transverse electromagnetic mass, the predictions of the various theories were (Fig. 3): Experiments of 1905 In order to make a decision between those theories, Kaufmann again performed his experiments with higher precision. Kaufmann believed that he had conclusively disproven the