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Every chemistry student learns that J. J. Thomson discovered the electron in 1897. Fewer learn where the word came from. It was coined six years earlier by George Johnstone Stoney, an Irish physicist who had argued since 1874 that electricity comes in fixed little packets, and who estimated the size of one using nothing more than electrolysis and kinetic theory. This post covers who he was, his argument, his calculation redone step by step, and what you can learn from it.
Who was George Johnstone Stoney?
The Dictionary of Irish Biography gives his birth as 15 February 1826 at Oakley Park near Birr, in what is now County Offaly, and his death in London on 5 July 1911. After the famine years, when the family estate was sold, he and his brother paid their way through Trinity College Dublin by private tutoring. From 1848 he was astronomical assistant to the Earl of Rosse at Birr Castle, home of what was then the largest telescope in the world. He was professor of natural philosophy at Queen's College Galway, then for 25 years secretary of the Queen's University in Ireland, doing his research in his spare time. He was elected a Fellow of the Royal Society in 1861 and was a strong supporter of university education for women; two of his daughters worked in physics and medical electricity in London.
What did he notice about electrolysis?
In a paper first read to the British Association at Belfast in 1874 and printed in the Philosophical Magazine in May 1881, "On the Physical Units of Nature", Stoney looked for units that nature itself supplies rather than the metre or the gram. He picked three: the speed that links electric and magnetic units (the speed of light), the gravitational constant, and a charge from electrolysis. He restated Faraday's law this way: "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." That fixed quantity he called E₁.
If every broken bond carries the same charge, then charge is not a smooth fluid. It comes in units.
Worked example: Stoney's estimate, step by step
His 1881 paper works in powers of ten, because he was not even sure of those. Here is his chain.
Molecules in a gas. Earlier estimates, including Josef Loschmidt's 1865 work and his own 1868 paper, suggested about 10¹⁸ molecules per cubic millimetre, so about 10²⁴ per litre.
Mass of a hydrogen atom. A litre of hydrogen weighs roughly a decigram, 0.1 g. So one molecule is about 0.1 / 10²⁴ = 10⁻²⁵ g. The atom is half that, but he kept 10⁻²⁵ g, since he was unsure of the power of ten anyway.
Hydrogen per unit of charge. Experiment showed that each "ampere" of electricity (in 1874 this word meant the quantity we now call a coulomb) decomposes 92 millionths of a gram of water, containing about 10⁻⁵ g of hydrogen.
His unit charge. His metric unit of electricity was 100 of those, which frees 10⁻³ g, one milligram, of hydrogen. That is 10⁻³ / 10⁻²⁵ = 10²² atoms, one bond broken per atom. So each bond carries 100 / 10²² = 10⁻²⁰ coulomb.
Today, e = 1.602 × 10⁻¹⁹ C, so his value was about 16 times too small. The electrolysis and Faraday's laws post covers the modern version: e = F / N_A.
Why was it low? The chain is sound; the weak link is step 1. Today's Loschmidt constant gives 2.69 × 10¹⁶ molecules per cubic millimetre at 0 °C and 1 atm, about 37 times fewer than 10¹⁸. Too many molecules makes each atom too light, and so the charge per atom too small. Rerun step 4 with modern figures, 1.045 × 10⁻⁵ g of hydrogen per coulomb and an atom of 1.674 × 10⁻²⁴ g, and you get 1.60 × 10⁻¹⁹ C.
When did "electron" get its name?
In 1891 Stoney published a long paper on the cause of double lines in gas spectra in the Scientific Transactions of the Royal Dublin Society (volume 4). On page 583 he recalls his estimate, says a charge of this amount is linked with each chemical bond in an atom, and writes: "These charges, which it will be convenient to call electrons, cannot be removed from the atom; but they become disguised when atoms chemically unite." He thought the motion of these charges inside molecules produced the lines of a spectrum.
In a 1894 letter to the Philosophical Magazine he defended his priority over Hermann von Helmholtz, pointing out that Helmholtz's similar statement, in his Faraday Lecture of April 1881, came after his own two announcements.
What mistakes do students make here?
"Stoney discovered the electron." He named and estimated a unit of charge. He did not detect a free particle. In his 1906 Nobel lecture, Thomson called the particles he found "corpuscles"; the name electron was attached to them later. See our post on J. J. Thomson and the electron.
Old units. In Stoney's 1874 paper "ampere" means a quantity of charge, not a current. He pointed this out himself in 1894.
Mixing up e and F. F = 96,485 C is the charge on a mole of electrons; e is the charge on one.
If you want to see the arithmetic of e = F / N_A explained one step at a time, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning.
What can a student take from how he worked?
Restate a law until it says something new. Faraday's law was well known. Stoney's rewording, "the same for each bond", turned it into a statement about atoms of charge.
Estimate when you cannot measure. Powers of ten were enough to show the charge was tiny but finite.
Do science around a day job. He did most of this as a university administrator.
Next step: redo Stoney's four steps with 10¹⁷ molecules per cubic millimetre and see how close you get to 1.6 × 10⁻¹⁹ C.
