and from , when combined according to scheme (6), we obtain , which is called the "magnetic rest force":
(8)
The vectors "electric rest force and magnetic rest force" are connected with , which determine the ponderomotive forces on electric and magnetic poles in motion, and which I wrote down in the first paper as and :
(9)
Evidently we have:
(9a)
(9b)
setting
(9c)
From the two
and
which are composed according to formula (1a), we obtain the :
By insertion of :
(10a
the last can be written:
(11)
By multiplying the by the speed of light (), we obtain the "relative radius" vector of my first paper [l. c.), equation IV].
Finally we combine which is represented by (11), with the -"velocity" according to scheme (6). is calculated as follows:
When multiplied by (), we arrive at the -"rest radius" of Minkowski:
(12)
§ 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 :