is determined. We then obtain for the complete stress tensor,
The heuristic value of the theory of invariants, which arises from the isotropy of space (equivalence of all directions), becomes evident from this example.
We consider, finally, Maxwell's equations in the form which are the foundation of the electron theory of Lorentz.
is a vector, because the current density is defined as the density of electricity multiplied by the vector velocity of the electricity. According to the first three equations it is evident that
is also to be regarded as a vector. Then