Max Abraham

“ However, he thought it necessary to add an additional force to the ponderomotive electromagnetic force, which is incompatible [4] with my system of electrodynamics. In § 4 I will write the equations of motion so that they satisfy the principle of relativity, without introducing the additional force of Minkowski. However, it must be admitted that the "rest density" of mass is not constant, yet it increases every time, when an electric current generates heat (in Joule) in matter; this hypothesis was already before stated by A. Einstein and M. Planck in relation to the principle of relativity. ”

“ In the electrodynamics of moving bodies, the equations of the momentum and energy apply: [8]
In order to give to these four equations a more symmetrical from, we put:
Then they become: Now, in Minkowski's theory, the system of four variables
,
where the first three are the components of which determines the ponderomotive force per unity of space, must form a . The 16 variables , from which that system is derived using equations (16) , must be transformed in such a way that this condition is satisfied. ”

“ When multiplied by () , we arrive at the -"rest radius" of Minkowski:
§ 3. Four-dimensional tensors .
A "Four-dimensional tensor" () is a set of ten variables, which are transformed into the orthogonal Lorentz transformations, such as we transform the squares and products of the coordinates :
When projecting into the space of three-dimensional () , then the first six components form a three-dimensional tensor () , which are transforming as the squares and products of () ; the following three components of are forming a , the tenth a scalar . ”