By Timothy M M Baker
One of the most significant physics experiments of the nineteenth century took place in 1868 at The Lawn, John Peter Gassiot’s house on Clapham Common South Side. The experiment was done by the great Scottish theorist of electromagnetism, James Clerk Maxwell, in order to help verify his contention that light is an electromagnetic phenomenon.
Clerk Maxwell was a visitor to Clapham for the experiment. His host was John Peter Gassiot, who had lived at The Lawn since 1836. The house, which stood at No 63 South Side, opposite the Windmill Inn, was built in 1746 and described in James Edwards’s A Companion from London to Brighthelmston, in Sussex, 1801: ‘a white house, a good building, but low. The window frames are of stone.’ It was demolished in 1902 to make way for the Clapham High School for Girls, itself replaced in the 1980s by St Gerard’s Close.
Gassiot was a wine and tobacco merchant in the City of London, importing port, sherry, and cigars. He also had interests in banking, insurance, and local government. But he was most famous as an electrical experimenter, who specialised in the electric arc, the discharge of electricity through air, which was the first widespread method of electric lighting, before Joseph Swan’s invention of the incandescent light bulb. Gassiot provided demonstrations of electric lighting and other electrical phenomena, both at Clapham and in London. A Fellow of the Royal Society, he conducted research on the electric arc at The Lawn, for which purpose he installed in the house a series of powerful electric batteries to provide the necessary high voltages.
It was the presence of these batteries that attracted other scientists to Clapham in order to use them for high-voltage experiments. Clerk Maxwell used Gassiot’s 2600-cell, 3000-volt mercury dichloride battery for his experiment to measure the ratio of electrodynamic
to electrostatic units of electricity.
In the 1830s, 1840s, and 1850s, at the Royal Institution in Westminster, Michael Faraday, a friend of Gassiot’s, had conducted extensive research on the relationship between electricity and magnetism. From this work, Faraday developed the theory of the
electromagnetic field, replacing previous notions that electric and magnetic forces propagated across space instantaneously at a distance. In the 1850s and early 1860s James Clerk Maxwell, successively a professor at Marischal College, Aberdeen, and King’s College, London, and later at his country house in Kirkcudbright, developed Faraday’s ideas mathematically into the concept of a ‘flux’ of electromagnetic energy, and generated ‘Maxwell’s Equations’, which describe the interaction between the ‘curl’ of moving electric
and magnetic fields, and the ‘divergence’ of static electric and magnetic fields.
Maxwell’s Equations predicted that a changing electromagnetic field travels as a self-sustaining wave, whose speed, representing the ratio between electrodynamic and electrostatic units, corresponded to the observed speed of light. Clerk Maxwell thus identified light as an electromagnetic phenomenon. The purpose of his experiment using Gassiot’s battery at Clapham Common was to measure the ratio of the units as accurately as possible, and thus to verify its correspondence to the speed of light. The German physicists Wilhelm Weber and Rudolf Kohlrausch had measured the ratio in 1856 as within a few percentage points of the speed of light, but thought this was coincidence. Clerk Maxwell was convinced by the theory that the correspondence was real.
In order to measure the ratio, Clerk Maxwell constructed at The Lawn a torsion balance: a modified version of the same type of instrument, invented by John Michell, that Charles Cavendish had used to ‘weigh the Earth’ at Cavendish House, nearby on Clapham
Common, in 1798 (see Derrick Johnson’s article, Clapham Society Local History Series 31, 2018). The torsion balance weighs opposing forces by the horizontal twisting of a beam suspended from a wire by its middle. Clerk Maxwell used it to weigh (1) the electrostatic
attraction between two charged metal discs, which were held by Gassiot’s battery at a specific voltage difference; against (2) the electrodynamic repulsion between two current-carrying coils. Assisted by Charles Hockin of St John’s College, Cambridge, Clerk Maxwell adjusted the electrical resistance in the circuit until the attraction between the charged discs and repulsion between the current-carrying coils balanced each other. The resistance at this equilibrium then corresponded to the ratio of forces, whose mathematical form is a velocity: the velocity of the ‘virtual photons’ that mediate the electromagnetic force.
Averaging the results of twelve separate experiments, Clerk Maxwell arrived at a mean measured value for the equilibrium resistance of 28.798 ohms, which corresponded to a ratio of electrodynamic and electrostatic units of 287,980,000 metres per second. This result was 7.3% below the ratio that Weber and Kohlrausch had measured in 1856, and 3.4% below the most accurate direct measurement at the time of the speed of light in air, which had been made by French physicist Léon Foucault in 1862. The actual speed of light in a vacuum is 299,792,458 metres per second, 4.1% more than Clerk Maxwell’s measurement of the ratio.
The discrepancy was not surprising, given the scope for experimental error. However, the fact that the figures were of the same order of magnitude gave important support to Clerk Maxwell’s theory. Clerk Maxwell’s experiment at The Lawn was thus effectively the first measurement of the speed of light to be informed by a theoretical understanding of what light is, and why it behaves as it does. Clerk Maxwell’s theory launched the study of light as an electromagnetic interaction with matter. The fact that in the theory the speed of light is a constant went on to become the foundation for the early twentieth century’s fundamental reevaluation of the nature of physics: the starting point for Albert Einstein’s theories of relativity, for quantum field theory, and for the modern ‘Standard Model’ of particle physics. As Einstein wrote on the centenary of Clerk Maxwell’s birth in 1931: ‘This change in the conception of reality is the most profound and the most fruitful that physics has experienced since Newton’. In 1964 the theorist of quantum electrodynamics, Richard
Feynmann, opined that ‘From a long view of the history of mankind … the most significant event of the nineteenth century will be judged as Maxwell’s discovery of the laws of electrodynamics’.
Bibliography
- Baker, Timothy M M: ‘Clapham Common constants: Henry Cavendish, James Clerk Maxwell, gravitation, and the speed of light’, The Antiquarian Astronomer, 2024.
- Clerk Maxwell, James: ’On Faraday’s Lines of Force’, Transactions of the Cambridge Philosophical Society, X, 1 (1855-1856), 155-229.
- Clerk Maxwell, James: ’On Physical Lines of Force’, Philosophical Magazine 90 (1865), 11-23.
- Clerk Maxwell, James: ‘A Dynamical Theory of the Electromagnetic Field’, Philosophical Transactions of the Royal Society of London 155 (1865), 459-512.
- Clerk Maxwell, James: ’On a Method of Making a Direct Comparison of Electrostatic with Electromotive Force; with a Note on the Electromagnetic Theory of Light’, Philosophical Transactions of the Royal Society of London, 158 (1868), 643-657.
- Clerk Maxwell’s summary of the experiment is in Proceedings of the Royal Society, 16, 16 (1868), 449-450.
- Harrison, WH: Article on John Peter Gassiot in Dictionary of National Biography, 2004, revised by Iwan Rhys Morus, 2011.
- Hunt, Bruce J: Imperial Science: Cable Telegraphy and Electrical Physics in the Victorian British Empire, 2021.
- Mahon, Basil: The Man Who Changed Everything, 2003.