Page:The Meaning of Relativity - Albert Einstein (1922).djvu/59

Rh There is, therefore, a four-dimensional vector of force per unit volume, whose first three components, $$k_1,k_2,k_3$$, 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 $$\sqrt{-1}$$.

A comparison of (36) and (35) shows that the theory of relativity formally unites the ponderomotive force of the electric field, $$\rho\mathbf{e}$$, and the Biot-Savart or Lorentz force $$[\mathbf{i}, \mathbf{h}]$$.