Variation of Mass with Velocity: "Kugeltheorie" or "Relativtheorie"
  This paper deals with four topics: The first subject is Abraham's spherical electron, Lorentz's contracted electron and B\"ucherer's electron. The second topic is Einstein's 1905 relativity theory of the motion of an electron. Einstein obtained expressions for the longitudinal and transverse masses of the electron using the principle of relativity and that of the constancy of the velocity of light. The third topic is Einstein's reply to Ehrenfest's query. Einstein's above solution appeared to Ehrenfest very similar to Lorentz's one: a deformed electron. Einstein commented on Ehrenfest's paper and characterized his work as a theory of principle and reasoned that beyond kinematics, the 1905 heuristic relativity principle could offer new connections between non-kinematical concepts. The final topic is Kaufmann's experiments. Kaufmann concluded that his measuring procedures were not compatible with the hypothesis posited by Lorentz and Einstein. However, unlike Ehrenfest, he gave the first clear account of the basic theoretical difference between Lorentz's and Einstein's views. Finally, B\"ucherer conducted experiments that confirmed Lorentz's and Einstein's models; Max Born analyzed the problem of a rigid body and showed the existence of a limited class of rigid motions, and concluded, "The main result was a confirmation of Lorentz's formula". 
Galina Weinstein
physics.hist-ph