POYNTING, JOHN HENRY

(b. Monton, near Manchester. England, 9 September 1852; d. Birmingham, England, 30 March 1914)

physics.

The youngest son of a Unitarian minister, Poynting attended his father’s school and then Owens College, Manchester (1867-1872). In 1872 he received the B.Sc. from London University. He studied at Trinity College, Cambridge, from 1872 to 1876 and was third wrangler on the mathematical tripos in 1876. He returned to Owens College as demonstrator in physics under Balfour Stewart. Made a fellow of Trinity in 1878, Poynting returned to Cambridge and did research in the Cavendish Laboratory, under the direction of Maxwell. He was appointed professor of physics at Mason College, Birmingham, on its founding in 1880 and remained there for the rest of his life. When Mason College became the University of Birmingham in 1900, Poynting became dean of the Faculty of Science, a position he held for twelve years. He received the Sc.D. from Cambridge in 1887 and became a fellow of the Royal Society the following year. His best-known work, the derivation of the expression for the flow of energy in an electromagnetic field (Poynting flux), appeared in 1884.

In 1893 Poynting was awarded the Adams Prize of Cambridge for his essay The Mean Density of the Earth. He was president of the Physical Society in 1905, a member of the Council of the Royal Society in 1909-1910, and vice-president of the Royal Society in 1910-1911. His honors included the Hopkins Prize of the Cambridge Philosophical Society (1903) and the Royal Society’s Royal Medal for 1905. Poynting married in 1880 and had three children. He loved the countryside where he lived, and was chairman of the Birmingham Horticultural Society; he also served as a Justice of the Peace. An excellent and beloved teacher, he died from a diabetic attack brought on by a bout of influenza.

From the time of his stay at the Cavendish Laboratory, Poynting performed painstaking experiments to measure the mean density of the earth or, equivalently, the constant of universal gravitation. Instead of using a torsion balance, as had Cavendish a century before, Poynting employed a beam balance. His best result, reported in 1891, differs from the presently accepted value by about four parts in a thousand. He admitted, however, that the use of the quartz-fiber torsion balance by C. V. Boys for the same purpose had proved to be inherently more accurate; and he employed that instrument in his later experiments on radiation pressure. In research performed with assistants at Birmingham, Poynting placed small upper limits on any dependence of the gravitational force between crystals on their orientation, and on any effect of temperature on gravitation.

The Poynting flux was derived in his paper of 1884 from Maxwell’s electromagnetic field theory. The term flux appears in an equation representing the energy balance in a closed region (“Poynting’s theorem”), and the concept was applied to trace the flow of energy around a conducting wire, a discharging capacitor, and a voltaic cell, and in an electromagnetic wave. Other work in electricity included the design of electrical instruments and discussion of Lodge’s models for representing the electromagnetic field. Poynting also did experiments on radiation pressure. At the turn of the century, P. N. Lebedev, and E. F. Nichols and G. F. Hull demonstrated the pressure exerted normal to a material surface. In 1904, with the aid of his colleague Guy Barlow, Poynting measured the tangential stress when a beam of light was reflected at an angle from a partially absorbing surface. This effect of the momentum carried by radiation had the experimental advantage of being less masked than the normal pressure by forces exerted by the unevenly heated residual gas in the apparatus. Poynting also demonstrated the existence of a torque on a prism so arranged that a beam of light emerged parallel to, but shifted from, the line of incidence. Perhaps of greater importance was his work with Barlow on the recoil of a heated, radiating body as a result of its own radiation, which work formed the subject of the Bakerian lecture for 1910.

The effect of radiation pressure on dust in the solar system, and the use of the law for the intensity of radiation from a blackbody to estimate the temperature of the planets, also interested Poynting. Other theoretical work involved more thorough discussion of the phase transition between the solid and liquid states (1881) and of osmotic pressure (1896). He developed instruments for research and for lecture demonstration, and performed research confirming his own predictions concerning the behavior of loaded wires under torsion (1905, 1909). Among Poynting’s earliest works were statistical studies on drunkenness in England (1877, 1878) and on fluctuation of commodity prices (1884). Besides his Adams Prize essay he wrote The Pressure of Light and The Earth, and was coauthor with J. J. Thomson of a series of physics textbooks.

BIBLIOGRAPHY

Poynting’s Collected Scientific Papers, G. A. Shakespear and G, Barlow, eds. (Cambridge, 1920), contains a bibliography and lists his books, of which the Adams Prize essay, The Mean Density of the Earth (London, 1894), and The Earth: Its Shape, Size, Weight and Spin (Cambridge, 1913) were most significant.

The Collected Scientific Papers contains several biographical notices, particularly one by J. J. Thomson from Proceedings of the Royal Society, 92A (1915-1916), i-ix.

A. E. Woodruff