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In the theoretical treatment of these electrons, we are faced

with the difficulty that electrodynamic theory of itself is

unable to give an account of their nature. For since electrical

masses of one sign repel each other, the negative electrical

masses constituting the electron would necessarily be scattered

under the influence of their mutual repulsions, unless there are

forces of another kind operating between them, the nature of

which has hitherto remained obscure to us.[13] If we now assume

that the relative distances between the electrical masses

constituting the electron remain unchanged during the motion of

the electron (rigid connection in the sense of classical

mechanics), we arrive at a law of motion of the electron which

does not agree with experience. Guided by purely formal points of

view, H. A. Lorentz was the first to introduce the hypothesis

that the form of the electron experiences a contraction in the

direction of motion in consequence of that motion.