He is best known for the first experimental proof of the velocity dependence of mass, which was an important contribution to the development of modern physics, including special relativity.

Of Jewish descent,[1] in 1890/1891, Kaufmann studied mechanical engineering at the Technische Hochschulen in Chalottenburg and Munich.

It is notable that Kaufmann himself interpreted his experimental results as confirmation of Abraham's theory, refuting the Lorentz-Einstein principle of relativity.

[4] The experiments were repeated by Alfred Bucherer (1908), Neumann (1914), and others, with results which appeared to confirm the Lorentz-Einstein theory and to disprove that of Abraham.

Even in 1917, investigations of the fine structure of the hydrogen lines provided confirmation of the Lorentz-Einstein formula, contradicting that of Abraham.