The electromagnetic field within the electron will vanish on the average \index{Acceleration of light-pulse!determined by symmetrical condition}% if it has sufficient symmetry. There appears to be an analogy between this \index{Symmetry!of an electron}% and the condition which we found in \SecRef{56} to be necessary for the existence of a particle, viz.\ that its gravitational field should have symmetrical properties. There is further an analogy in the condition determining the acceleration in the two cases. An uncharged undisturbed body takes such a course that relative to it there is no resultant gravitational field; similarly an electron takes such a course that relative to it there is no resultant electromagnetic field. We have given a definite reason for the gravitational symmetry of a particle, viz.\ because in practical measurement it is itself the standard of symmetry; I presume that there is an analogous explanation of the electrical symmetry of an electron, but it has not yet been formulated. The following argument (which should be compared with \SecRefs{64}, \SecNum{66}) will show where the difficulty occurs. The analogue of the interval is the flux $F_{\mu\nu}\, dS^{\mu\nu}$. As the interval between \index{Flux!electromagnetic}% two adjacent points is the fundamental invariant of mechanics, so the flux through a small surface is the fundamental invariant of electromagnetism. Two electrical systems will be alike observationally if, and only if, all corresponding fluxes are equal. Equality of flux can thus be tested absolutely; and different fluxes can be measured (according to a conventional code) by apparatus constituted with electrical material. From the flux we can pass by mathematical processes to the charge-and-current vector, and this enables us to make the second contact between mathematical theory and the actual world, viz.\ the identification of electricity. We should now complete the cycle by showing that with electricity so defined apparatus can be constructed which will measure the original flux. Here, however, the analogy breaks down, at least temporarily. The use of electricity for measuring electromagnetic fluxes requires discontinuity, but this discontinuity is obtained in practice by complicated conditions such as insulation, constant contact differences of potential, etc. We do not seem able to reduce the theory of electrical measurement to direct dependence on an innate discontinuity of electrical charge in the same way that geometrical measurement depends on the discontinuity of matter. For this reason the last chain of the cycle is incomplete, and it does not seem permissible to deduce that the discontinuous unit of electric charge must become the standard of electrical symmetry in the same way that the discontinuous unit of matter (turned in different orientations) becomes the standard of geometrical symmetry.