Kaufmann–Bucherer–Neumann experiments
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.
[2] 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.
[A 1][A 2] 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.
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.
[A 4][A 5] 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.
Therefore, Abraham introduced the "transverse electromagnetic mass" with the following velocity dependence: Kaufmann also made a calculation mistake in deriving the deflection curves.
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.
[10] 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.
[A 8][A 9][5][14] 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.
[A 10][15] 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.
[A 14][A 15] Shortly after Kaufmann published his results and the conclusions of his analysis, Max Planck decided to re-analyze the data obtained by the experiment.
[20] The main problem of Kaufmann's experiments was his use of parallel magnetic and electric fields, as pointed out by Adolf Bestelmeyer (1907).
Using a method based on perpendicular magnetic and electric fields (introduced by J. J. Thomson and further developed to a velocity filter by Wilhelm Wien), Bestelmeyer obtained considerably different values for the charge-to-mass ratio for cathode rays up to 0.3c.
A radium beta source was placed at the center of a circular condenser consisting of two silvered glass plates spaced 0.25 mm apart and charged to about 500 volts, set in a homogeneous 140 Gauss magnetic field.
[22][23] For his final analysis, Bucherer recalculated the measured values of five runs with Lorentz's and Abraham's formulas 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).
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.
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.
[25] 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.
[A 20][A 21][A 22] Lorentz summarized these efforts in 1915:[A 23] Zahn & Spees (1938)[36] and Faragó & Lajos Jánossy (1954)[37] argued that many assumptions employed in those early experiments as to the nature and the properties of electrons and the experimental setup, were wrong or imprecise.
[A 24][A 25] While the results of those electron deflection experiments were disputed for a long time, the investigations of the fine structure of the hydrogen lines by Karl Glitscher (based on the work of Arnold Sommerfeld) had already in 1917 provided a clear confirmation of the Lorentz–Einstein formula, because the relativistic expressions for momentum and energy were necessary to derive the fine structure, and a constituted a refutation of Abraham's theory.
[38][A 26] In addition, the first electron deflection experiments with sufficient precision was conducted by Rogers et al. (1940), who developed an improved setup.
The earlier measurements by Kaufmann, Bucherer, and others had used flat parallel plate condensers which provided no focusing of the beta particles.
10) instead constructed an electrostatic spectrograph capable of resolving the energy maxima of individual beta particle lines from the radium decay series.
