(58) | , |

*i,e.*

.

The vector fulfills the relation

(59) | , |

which we can write as

and is also *normal* to *w*. In case , we have , and

(60) | , |

I shall call , which is a space-time vector 1st kind the *Rest-Ray*.

As for the relation E), which introduces the conductivity , we have

This expression gives us the *rest-density* of electricity (see §8 and §4). Then

(61) |

represents a space-time vector of the 1st kind, which since , is *normal* to *w*, and which I may call the *rest-current*. Let us now conceive of the first three component of this vector as the *x-, y-, z* co-ordinates of the space-vector, then the component in the direction of is

and the component in a perpendicular direction is .

This space-vector is connected with the space-vector , which we denoted in § 8 as the conduction-current.

Now by comparing with , the relation (E) can be brought into the form

(E) | . |