Now if we make use of (59), and denote the space-vector which has as the x-, y-, z-components by the symbol , then the third component of 92) can be expressed in the form
The round bracket denoting the scalar product of the vectors within it.
§ 14. The Ponderomotive Force.
Let us now write out the relation in a more practical form; we have the four equations
It is my opinion that when we calculate the ponderomotive force which acts upon a unit volume at the space-time point x,y,z,t, it has got x-, y-, z-