from some fixed plane by , the line integral of the M.I. is , while the current, being an alteration of displacement, is

Therefore

(2) |

But since the displacement is propagated unchanged with velocity , the displacement now at a given point will alter in time to the displacement now a distance behind, where .

Therefore

(3) |

Substituting in (2)

whence

(4) |

the function of the time being zero, since and are zero together in the parts which the wave has not yet reached.

If we take the line-integral of the E.M.I. round a face perpendicular to the M.I. and equate this to the decrease of magnetic induction through the face, we obtain similarly

(5) |

It may be noticed that the product of (4) and (5) at once gives the value of , for dividing out we obtain

or

But using one of these equations alone, say (4), and substituting in (1) K for and dividing by , we have

or

whence