The first two terms are contributions of the *aether* in the sense of Lorentz's theory, the two latter ones are to be seen as contributions of polarized *matter* to the electromagnetic momentum density.

We begin to calculate the energy current. With respect to (32) and (31), the expression

(34) |

holds in Lorentz's theory for the vector introduced at the end of § 5. According to (31) and (30), we have

so that equation (21) assumes the form

Now, since it is to be set according to (34)

then eventually if follows as the *value of the energy current*

(35) |

The first term can be interpreted as a contribution of the *aether*, the second one as the contribution of the electrically *polarized matter* at the energy current, as G. Nordström^{[1]} has shown in a recently published paper, which is remarkable also in other respects; the third term which is added at the motion of magnetically polarized matter, corresponds in such a way to the second one, as it is required by the symmetry of electric and magnetic vectors assumed at this place.

§ 9. **Theory of H. Minkowski.**

In this theory, the following relations between the electromagnetic vectors hold

(36) |

(37) |

Also here, besides the vector pairs contained in the main equations, a new vector pair is added, which mediates the relation between them.

- ↑
*G. Nordström, Die Energiegleichung für das elektromagnetische Feld bewegter Körper*(Dissertation, Helsingfors 1908).