The Meaning of Relativity / Four lectures delivered at Princeton University, May, 1921

by Albert Einstein

There is, therefore, a four-dimensional vector of force per unit volume, whose first three components,

,

,

, are the ponderomotive force components per unit volume, and whose fourth component is the rate of working of the field per unit volume, multiplied by

.

A comparison of (36) and (35) shows that the theory of relativity formally unites the ponderomotive force of the electric field,