The Principle of Conservation of Motion of the Center of Gravity and the Inertia of Energy, A. Einstein, Annalen der Physik 20 (1906): 627-633
In a paper I published last years I showed that Maxwell's equations in conjunction with the principle of relativity and the principle of energy conservation led to the conclusion that the mass of a body changes with the changes with the change in energy content led to the conclusion that the mass of a body changes its energy content, no matter what kind of change of energy this may be. It turned out that to an energy change of magnitude DE there must correspond a change of mass of the same sign and of magnitude DE/V2 , where V denotes the velocity light.
In the present paper I want to show that the above theorem is the necessary and sufficient condition for the law of the conservation of the center of gravity to be valid (at least in the first approximation) also for systems which not only mechanical, but also electromagnetic processes take place. Although the simple formal considerations that have to be carried out to prove this statement in the main already contained in a workby H. Poincaré, for the sake of clarity I shall not base myself upon that work.