WHAT IS ELECTRICITY"? — HEYL 221 



era lasted several millenniums and the fluid theory three centuries, 

 the ethereal era lasted only a few decades. The fourth era is that 

 which is still with us. It may be called the atomic or quantum 

 period, in which it is noteworthy that but little attention has been 

 paid to the ultimate nature of electricity and a great deal to its 

 structure. It is difficult to say when this period began, as, in fact, 

 the ethereal era began to die almost as soon as it began to live. 



Wilhelm Weber,^^ in 1871, in developing his theory of magnetism, 

 pictured to himself light positive charges rotating about heavy nega- 

 tive ones, much like a satellite about a planet; and in 1874 Johnstone 

 Stoney read before Section A of the British Association a paper 

 entitled " The Physical Units of Nature ", which was not printed 

 until 7 years later.^^ In this paper he asserted the atomic nature of 

 electricity and made a rough calculation of the elementary charge 

 on the basis of Faraday's law of electrolysis. Ten years later ^^ he 

 was the first to use the term " electron." 



Helmholtz,^* in his Faraday lecture at the Royal Institution in 

 1881, further developed this line of thought, saying (p. 290) : 



Now the most startling result of Faraday's law is perhaps this. If we accept 

 the hypothesis that the elementary substances are composed of atoms, we cannot 

 avoid concluding that electricity also, positive as well as negative, is divided 

 into definite elementary portions, which behave like atoms of electricity. 



Maxwell himself saw that his electromagnetic theory was essen- 

 tially continuous in its nature, and recognized the difficulty arising 

 from the implications of Faraday's experiments. In his " Treatise 

 on Electricity and Magnetism" (vol. 1, p. 313, ch. 4, 1873), in 

 the chapter on electrolysis he says : 



It is extremely improbable that when we come to understand the true 

 nature of electrolysis we shall retain in any form the theory of molecular 

 charges. 



For Helmholtz, however, the atomic nature of electricity was be- 

 yond question. Electricity, as he saw it, was a special chemical ele- 

 ment ^^ whose atoms combine with those of other elements to form 

 ions. Moreover, it appeared to be a monovalent element, for it 

 seemed that a monovalent element combined with one electron, a 

 bivalent one with two, and so on, exactly as a chlorine atom combines 

 with one atom of hydrogen and an oxygen atom with two atoms of 

 hydrogen. Helium, with its zero valence and double electrical 

 charge, was as yet unknown. 



"Millikan, The Electron (2d ed.), p. 20, University of Chicago Press, 1924. 



" Stoney, Phil. Mag., vol. 11, pp. 381-390, 1881. 



" Stoney, Sci. Trans. Roy. Dublin Soc, 11th ser., vol. 4, p. 563, 1891. 



"Helmholtz, Journ. Chem. Soc. (London), vol. 39, pp. 277-304, 1881. 



" Graetz, Recent Developments in Atomic Theory, Methuen & Co., London, 1923, 



