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Stoney's Electron HISTORY OF PHYSICS 159 and accordingly he quoted that committee in determining that an Ampère-second (or Stoney's Coulomb) causes dissociation of 92 x to 6 gm of H20. With oxygen having atomic weight 16 and hydrogen atomic weight 1, Electron Stoney estimated that this quantity of Patrick A.Wayman water would contain: 92 x 1o-6 x (2/16) x Dublin Institute for Advanced Studies (1/1025) = x.15 x 1020 H atoms. Rounding this to 1020, Stoney deduced George Johnstone Stoney, the man who that one chemical bond would correspond named the electron, was led to the concept to 1o-20 Coulombs (which he calls of a fundamental unit of electric charge by Ampères). However, without dropping a combining electro-chemistry with the factor of 2, as explained above, and withou kinetic theory of gases. But his understand­ other approximation, he could have ing was incomplete (so the discovery of the arrived at the figure of 4.35 x 1o-21 electron is associated with JJ Thomson's Coulombs for his ‘unit of charge’. determination of its charge to mass ratio in The latter figure, as compared with a October 1897) modern figure for the charge on the elec­ The suggestion of the name ‘electron’ Stoney was able, by the 1870s, to com­ tron of 1.6018 x 1019 Coulombs, is low by a (amber in Greek) for a hypothetical small prehend a unit of electric charge being factor of 36.8 (not 16 as was 1o-20). In unit of electric charge is generally attrib­ exchanged in electro-chemical processes tracking down this discrepancy we notice uted to George Johnstone Stoney, FRS, involving molecules of water. The con­ Stoney’s use of the quantity 1o18 for the 1826-1911, born in Ireland, whose sister was stituents of these molecules were hypo­ number of molecules present in one cubic the mother of George Francis Fitzgerald, thetically the same atoms that were so millimetre of gas at standard temperature famous scion of a distinguished scientific vividly invoked in the kinetic theory of and pressure. Using Avogadro’s number family. Stoney’s career included work gases and, particularly, through informa­ 6.0238 x 1023, and the volume of a gm-mol- under Lord Rosse at Birr Castle and the tion on viscosity, where sizes of atoms, ecule (at s.t.p.) of 22.4146 x 106 mm3, we chair of Natural Philosophy at what was their number, and their mean free path derive, instead of 1018, the estimate then Queen’s College, Galway, before he were being estimated numerically. James 2.687 x 10l6, down by a factor of 37.22. If embarked on an administrative career that G. O’Hara, now of Hamburg University, Stoney had started his calculations with never quite suppressed his research. His pointed out in 1974 that Stoney’s derived this lower value his estimate, with all other many prescient publications tended to estimate of the ‘unit of charge’, assumptions except the dropped factor of : appear several years after the work had 10-20 Ampère (later called the Coulomb), unchanged, would have agreed with a been done and the ideas had been honed was 1/16th of the correct value of the modern value for the charge of the elec­ down to a final form. While the name of charge of the electron. Both O’Hara and tron within about 1%. Such agreement is the electron may have only first appeared the author of the Royal Society obituary spuriously close taking into account other in a paper of 1891, where it was linked to noted that in his estimate Stoney availed approximations; however, this circum­ hypothetical processes of oscillations tak­ himself of his determination of the num­ stance does not seem to have been noticed ing place among charged particles within ber of molecules present in one cubic mil­ previously, partly because Stoney’s delay atoms, Stoney’s estimate of the value of the limetre of gas at standard temperature and from 1874 to 1881 in publication caused ‘unit of electricity’ and the very concept of pressure: 1018. It turns out that a substantial him to bring in a number of side-issues the natural occurrence of such a unit, was error in this figure was the only apprecia­ that led to a confusing situation over his due to work completed in 1874 - being pre­ ble error in Stoney’s work. That his result estimates. It is apparent that his reasoning sented at the British Association for the should suffer from such an error in the was correct except that simplistic and Advancement of Science meeting at Belfast available data detracts nothing from the incorrect inferences had been drawn from in 1881. merit of Stoney’s performance. It was pio­ the kinetic theory of gases. Modern values According to Stoney’s Royal Society neer work in an obscure and difficult line for the charge on the electron come either obituary, Herman von Helmholtz indepen­ of research. from radiation theory (Planck’s Law and dently drew attention to the existence of the Boltzmann Constant) or by virtually definite elementary charges which behave With one litre of H2 gas weighing 0.1 gm, direct measurement in Millikan’s famous like atoms of electricity in 1881; also, it is Stoney estimated that one H2 molecule oil-drop experiment. During his produc­ well-known that Joseph John Thomson, in weighs 10-25 gm and that the chemical atom tive period Stoney was fully occupied pro­ October 1897, published an account regard­ is half of this. He expressed the opinion fessionally in university administration. It ed as the announcement of the discovery that there is no advantage in retaining the was only by rising at 5 am to work on sci­ of the electron (under another name) as a coefficient of a half in such an estimate, as ence and by association with the physics particle with a definite charge to mass we are not even sure that we have assigned ‘giants’ of Trinity College, Dublin (such as ratio. Even so, it was not until 1899 when the correct power of ten. He therefore took George Francis Fitzgerald, his nephew, he read a paper ‘On the existence of mass­ 10 25 gm as being the value of the mass of John Joly; Thomas Preston, and Robert es smaller than the atoms’ before the an atom of hydrogen. Ball) and other colleagues in the Royal British Association that his views really Stoney served for many years on a com­ Dublin Society, that he could produce his made headway outside the Cavendish mittee for defining electrical standards many valuable and highly ingenious con­ Laboratory. such as the Ampère, the volt and the ohm, tributions to physics and astronomy. News September/December Europhysics 1997.
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