According to~\Eq{(80.6)} the mass of the electron is~$e^{2}/4\pi a$, where $a$~is a length comparable with the radius of the electron. This is in conformity with the usual view as to the size of an electron, and is opposed to the point-electron suggested in \SecRef{78} as an alternative. But the mass here considered is a purely \PageSep{193} electromagnetic constant, which only enters into equations in which electromagnetic forces are concerned. When the right-hand side of~\Eq{(80.1)} vanishes, the electron describes a geodesic just as an uncharged particle would; but $m$~is now merely a constant multiplier which can be removed. We have still to find the connection between this electromagnetic mass \index{Mass!electromagnetic}% \[ m_{e} = e^{2}/4\pi a \Tag{(80.71)} \] and the gravitational (i.e.\ gravitation-producing) mass~$m_{g}$, given by \[ m_{g}\, ds = \frac{1}{8\pi} \int G\sqrt{-g}\, d\tau. \Tag{(80.72)} \] Since we believe that all negative electrons are precisely alike, $m_{g}/m_{e}$ will be a constant for the negative electron; similarly it will be a constant for the positive electron. But positive and negative electrons are structures of very different kinds, and it does not follow that $m_{g}/m_{e}$~is the same for both. As a matter of fact there is no experimental evidence which suggests that the ratio is the same for both. Any gravitational field perceptible to observation is caused by practically equal numbers of positive and negative electrons, so that no opportunity of distinguishing their contributions occurs. If, however, we admit that the principle of conservation of energy is universally valid in cases where the positive and negative electrons are separated to an extent never yet realised experimentally, it is possible to prove that $m_{g}/m_{e}$~is the same for both kinds. From the equation~\Eq{(80.1)} we deduce the value of the electromagnetic energy-tensor as in \SecRefs{76},~\SecNum{77}; only, $E^{\mu\nu}$~will not be expressed in the same units as the whole energy-tensor $G_{\mu}^{\nu} - \frac{1}{2} g_{\mu}^{\nu} G$, since the mass appearing in~\Eq{(80.1)} is~$m_{e}$ instead of~$m_{g}$. In consequence, the law for empty space~\Eq{(77.6)} must be written \[ G_{\mu}^{\nu} - \tfrac{1}{2} g_{\mu}^{\nu} G = -8\pi\, \frac{m_{g}}{m_{e}} (-F^{\nu\alpha} F_{\mu\alpha} + \tfrac{1}{4}g_{\mu}^{\nu} F^{\alpha\beta} F_{\alpha\beta}). \Tag{(80.8)} \] We can establish this equation firstly by considering the motion of a positive electron and secondly by considering a negative electron. Evidently we shall obtain inconsistent equations in the two cases unless $m_{g}/m_{e}$ for the positive electron is the same as for the negative electron. Unless this condition is fulfilled, we should violate the law of conservation of energy and momentum by first converting kinetic energy of a negative electron into free electromagnetic energy and then reconverting the free energy into kinetic energy of a positive electron.