From the momentum equations (8), we find the expression for the performance of work of the ponderomotive force

If we set here for abbreviation

(13) |

then the energy equation (9) gives with respect to (12)

(14) |

This relation gained from the momentum and energy theorem, will prove itself to be still important.

§ 4. **The main equations.**

Common to all theories of electrodynamics of moving bodies, is the form of the two first main-equations

(I) |

(II) |

They are nothing else than a general scheme, which obtains a physical sense only by addition of two relations between the four arising vectors; and two such relations are necessary, to reduce the number of unknown vectors to two; the temporal change of the field of these two vectors, is then described by the two first main-equations.