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that as far as terms in the electric part of the energy is unaltered by the motion.

(C) Energy of a very slender Ellipsoid. When the ellipsoid is so slender that may be neglected in comparison with unity we have

. (26)

When is small, this becomes


(D) Energy of a Disk.

When the ellipsoid is more oblate than Heaviside's, and becomes negative. In this case let us write


so that is the radius of the disk which is the "image" of the ellipsoid . Then writing we have from (23)




so that (23) becomes

. (27)

When we find for the energy of a disk of radius moving along its axis

. (28)

In all these cases it will be found that when the energy becomes infinite, so that it would seem to be impossible to make a charged body move at a greater speed than that of light.