WHAT IS ELECTRICITY? — HEYL 227 



In 1893 this suji^gestion was of academic interest only, no bodies 

 moving with sufficient speed being then available for experiment. A 

 few years later conditions had changed. The study of radioactive 

 substances and of the discharge of electricity through gases had 

 placed at our disposal positively and negatively charged particles 

 moving with unprecedented speeds, which in the case of the negative 

 particles were in some cases comparable with the speed of light. 

 Here, it would seem, was an opportunity to test Thomson's theory 

 of increasing mass. 



Unfortunately, the conditions of the problem were such that it was 

 not at first possible to obtain a measure of the mass of such a par- 

 ticle, but only a determination of the ratio of the electric charge to 

 the mass which carried it (e/m). 



Kaufmann ^^ found, however, that for the swifter particles this 

 ratio was less than for the slower ones. There were only two ways 

 of explaining this fact, both equally radical: either the mass in- 

 creased or the charge diminished as the speed of the particle became 

 greater. 



In this dilemma opinion inclined generally to the first alternative, 

 largely because there was in existence a theoretical reason to expect 

 it, while no one as yet had been ingenious enough to suggest any 

 reason why a moving charge should alter. It is of importance to 

 note that Kaufmann's experimental result, because of its equivocal 

 character, cannot be accepted as more than half proving J. J. 

 Thomson's theory. 



Kaufmann calculated that such particles as he experimented with 

 might have, when moving slowly, an electrical mass equal to about 

 one-fourth their total mass. In making this calculation he assumed 

 that a particle behaved as though it were a little metallic conductor, 

 but he was careful to point out that a different assumption might 

 lead to another result. 



J. J. Thom3on, on the assumption that a particle had no metallic 

 conductivity, but acted like a point charge, found that Kaufmann's 

 results indicated that the whole of the mass of the particle might be 

 accounted for electrically. 



This was the origin of the electrical theory of matter. Its pedigree 

 goes back to J. J. Thomson's theory, which in turn was derived from 

 the electromagnetic theory of Maxwell. Kaufmann's experiments 

 only half proved Thomson's theory, which in addition was compli- 

 cated by a special assumption with regard to the distribution of the 

 charge on the particle. Without this assumption only a part of the 

 mass could be accounted for electrically. 



2=; Kaufmann, Gcsell. Wiss. GOttingen, Nov. 8, 1901; July 26, 1002; Mar. 7, 1903. 

 3692.'}— 36 16 



