Ebenezer Cunningham

“ If the change in the shape of the electron with the change in velocity is taken into account, it will be found that the mass as obtained from the change in momentum is identical with the mass as obtained from the change in the energy, as it clearly must be, since a quantity defined in a perfectly definite manner cannot from consistent equations be shown to have two different values. ”

“ Thus if the single electron at rest has a spherical configuration, and there are no other than electromagnetic forces, we should expect it in motion to have a spherical configuration when measured by the variables x' y' z' which means that as measured by the variables x, y, z it will have the spheroidal shape as suggested by Lorentz.
The electron as conceived by Abraham, on the other hand, is spherical always as regards the variables x, y, z, and a prolate spheroid as regards x' y' z' the ratio of the axes being . ”

“ Suppose axes of ξ η ζ to be B's system of coordinates moving with him with velocity v relative to A's axes of x y z, and coinciding with them at t=0.
Associated with a given point at a given time as marked by the values (x, y, z, t) will be unique values of (ξ, η, ζ, τ) , τ being B's measure of the interval elapsed from the time of his coincidence with A, and conversely. There must therefore be a linear transformation from the variables (x, y, z, t) to (ξ, η, ζ, τ) . ”