(65) K_{σ} = F_{σμ} J^μ.
According to (61) its components can be written down in the
three-dimensional notation.
{ K₁ = ρE_{_x_} + [_i_, H]_{x}
(65a) { — — —
{ K₄ = — (_i_, E).
K_{σ} is a covariant four-vector whose components are equal to the
negative impulse and energy which are transferred to the
electro-magnetic field per unit of time, and per unit of volume, by the
electrical masses. If the electrical masses be free, that is, under the
influence of the electro-magnetic field only, then the covariant
four-vector K_{σ} will vanish.
In order to get the energy components T_{σ}^ν of the electro-magnetic
field, we require only to give to the equation K_{σ} = 0, the form of
the equation (57).
From (63) and (65) we get first,