Einsteins akademischer Aufstieg
Part of an Experimental Setup by Walter Kaufmann
For studying changes in the mass of fast electrons, 1906
It follows from the theory of relativity that an electron's mass depends on its velocity. Other theories also predict such a change in mass, although the change is not the same size. Walter Kaufmann deflects electrons in a tube with a magnet. He can calculate an electron's mass from the curve of its path. His initial test results seem to contradict Einstein's theory.