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

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 $[\mathbf {i} ,\mathbf {h} ]$.