George Johnstone Stoney and the charge of the electron

George Johnstone Stoney FRS (1826-1911) was born in the townland of Oakley Park, Clareen, near Birr, Co. Offaly.  He was educated at home and then at Trinity College Dublin, where he was awarded a BA degree in 1848.  Following his degree he became an assistant to William Parsons the 3rd Earl of Rosse at Birr Castle, close to his childhood home in Co. Offaly. He competed for fellowship at TCD in 1851 but was unsuccessful. However, he was appointed to the chair of natural philosophy (mathematics) at Queen’s College Galway in 1852, through the influence of his mentor. In 1857 he returned to Dublin in an administrative post in the Queen’s University of Ireland (which was established in 1850, as the degree-awarding university of the Queen's Colleges of Belfast, Cork, and Galway that had been established in 1845). He continued in this post until 1882. 

Stoney moved to London in 1893 on his retirement.  He continued his scientific work throughout his working life and continued to publish his scientific ideas into the 1890’s.  Stoney was elected a Fellow of the Royal Society (FRS) in 1861 and Royal Society vice-president in 1898. In 1899 he was the first person to be awarded the Boyle medal by the Royal Dublin Society, Ireland’s most prestigious scientific award. George Francis Fitzgerald of TCD was the son of his sister, Anne Frances Stoney.

Stoney is best known as the originator of the name, ‘electron’, for the fundamental unit of electric charge. J. G. O’Hara has written an account of Stoney’s work [1] on the kinetic theory of gases, emission of radiation of specific wavelengths by molecules through interaction of molecular vibrations with the ether and, especially, systems of units and values of fundamental physical constants, which led to his naming of the electron and many other topics.

Stoney was a member for at least 40 years of a British Association (BA) committee on electrical standards which sat between 1862 and 1912.  Other distinguished members of the committee included William Thomson (Lord Kelvin) and James Clerk Maxwell [1]. In a paper first read at the Belfast meeting of the BA in 1874, Stoney introduced his own system of units based on three quantities which he designated, V1, G1 and E1.  V1 is a velocity and is equal to the speed of light in free space (then believed to be its velocity in the ether), G1 is the gravitational constant and E1 is the fundamental unit of electric charge, now the charge of the electron. 

Stoney’s estimate of the fundamental unit of charge was calculated using the amount of hydrogen gas released when water was ‘electrolysed’ by passing an electric current through it. Stoney’s work on the kinetic theory of gases had previously led him to estimate of the number of molecules contained in a cubic millimetre of a gas [2]. Then, making the assumption that the quantity of electric charge contained in the hydrogen chemical bond was the fundamental charge, measuring the volume of hydrogen gas released by electrolysis and the quantity of electric current passed through the electrochemical cell, he was able to deduce the magnitude of the electric charge per chemical bond. 

Regarding the unit of electric charge, Stoney commented [3], ‘And, finally, Nature presents us, in the phenomenon of electrolysis, with a single definite quantity of electricity which is independent of the particular bodies acted on. To make this clear I shall express ‘Faraday’s Law’ in the following terms, which, as I shall show will give it precision, viz—for each chemical bond which is ruptured within an electrolyte a certain quantity of electricity traverses the electrolyte which is the same in all cases. This definite quantity of electricity I shall call E1. If we make this our unit quantity of electricity we shall probably have made a very important step in our study of molecular phenomena. Now the whole of the quantitative facts of electrolysis may be summed up in the statement that a definite quantity of electricity TRAVERSES THE SOLUTION FOR EACH CHEMICAL BOND THAT IS SEPARATED’.

The value that he arrived at was 10-20 ampere (which was at that time a unit of charge rather than electric current, as it is today).  This was about one sixteenth of the modern value of 1.6 10-19 coulomb.  Millikan’s famous oil drop experiment, in which electric charges on microscopic oil drops were measured, gave a more accurate value (within 0.6% of the currently accepted value) and was performed in 1909.

Communication between scientific communities of important new results is not always perfect.  When the German physicist Hermann Ebert claimed in 1894 that, ‘Von Helmholtz, on the basis of Faraday’s law of electrolysis, was the first to show in the case of electrolytes that each valency must be considered charged with a minimum quantity of electricity, the ‘valency-charge,’ which like an electrical atom is no longer divisible.’, Stoney strongly defended his claim to priority in identifying the electron as the fundamental unit of charge on the basis of electrolysis. In a letter to Philosophical Magazine [4] in 1894 he stated that he had already ‘pointed out this remarkable fact’, first at the Belfast meeting of the British Association in August 1874 and had read the same paper again  before the Royal Dublin Society on 16 February 1881 and published the idea in the proceedings of that meeting and in Philosophical Magazine in May 1874 [3].  Helmholtz’s announcement was made in his Faraday Lecture on 5th April 1881, subsequent to Stoney’s addresses.

Stoney is best known for coining the name, ‘electron’, for the fundamental unit of electric charge.  However, his scientific work ranged over many branches of physical science including physical optics, solar physics and astronomy, atmospheric physics, acoustics and molecular physics [1]. He clearly had great physical insight and imagination.  This can be seen especially in his work on the origins of molecular spectra in which he proposed that electrons moving in elliptical orbits and interacting with the ether were responsible for emission of electromagnetic waves [5].  This last paper on the topic was published in 1891, two years after the publication [6],  by his nephew, G. F. Fitzgerald, of the idea that bodies moving through the supposed ether contract in the direction parallel to their motion via mechanical forces, which has has become known as the Fitzgerald-Lorentz contraction, although the origin of the effect is explained instead by Einstein’s special theory of relativity.

[1] J.G. O’Hara, George Johnstone Stoney, F.R.S. and the concept of the electron, Notes and Records of the Royal Society of London, 29, (2) (1975).

[2] The internal motions of gases compared with the motions of waves of light, G. J. Stoney, Phil.Mag., (4), 36,132-141, (1868).

[3] On the physical units of nature, G. J. Stoney, Phil. Mag., (5), II, 381-390, (1881).

https://www.tandfonline.com/doi/abs/10.1080/14786448108627031

[4] Of the "Electron," or Atom of Electricity, G. J. Stoney, Phil. Mag., (5), 38, 418-420, (1894).

[5] On the cause of double lines and equidistant satellites in the spectra of gases, G. J. Stoney, Trans. R. Dubl. Soc., 4, 563-608, (1888-1892).

[6] The ether and the earth’s atmosphere, G. F. Fitzgerald, Science 13, 390 (1889).