Dictionary of National Biography, 1927 supplement/Poynting, John Henry

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4167894Dictionary of National Biography, 1927 supplement — Poynting, John Henry1927Joseph John Thomson

POYNTING, JOHN HENRY (1852–1914), physicist, the youngest son of the Rev. T. Elford Poynting, Unitarian minister at Monton, near Manchester, by his wife Elizabeth Long, of Bath, was born at Monton 9 September 1852. He received his earlier education at the school kept by his father, and then went in 1867 to the Owens College, Manchester (now the university of Manchester). He took the B.Sc. degree at London University in 1872. In the same year he gained an entrance scholarship at Trinity College, Cambridge, and came into residence at Cambridge in October. He took his degree in 1876, being placed third in the list of the mathematical tripos. Immediately afterwards he went back to the Owens College and demonstrated in the physical laboratory under Balfour Stewart [q.v.]. On his election to a fellowship at Trinity College in 1878 Poynting returned to Cambridge, and began in the Cavendish laboratory, under James Clerk Maxwell [q.v.], experiments on the mean density of the earth which occupied much of his time for the next ten years. He remained at Cambridge until 1880, when he was elected to the chair of physics in Mason College, Birmingham (now the university of Birmingham), which had just been founded. This post he held until his death. In 1887 he received the Sc.D. of Cambridge and in 1888 he was made a fellow of the Royal Society; in 1893 the Adams prize was awarded to him, and the Hopkins prize in 1903. He was president of Section A of the British Association in 1899, and president of the Physical Society in 1905. In the latter year he received a royal medal from the Royal Society ‘for his researches in physical science, especially in connexion with the constant of gravitation and the theories of electro-dynamics and radiation’. He was a vice-president of the Royal Society in 1910–1911.

Poynting's most important contributions to physics are two papers communicated to the Royal Society: On the Transfer of Energy in the Electromagnetic Field (Philosophical Transactions, A, 1884), and On the Connexion between Electric Currents and the Electric and Magnetic Induction in the Surrounding Field (ibid., 1885). These papers revolutionized ideas about the motion of energy in the electric field. To take an example: before the publication of these papers, when a charged Leyden jar was discharged by connecting the inside and outside by a wire, the energy was supposed to travel along the wire much in the same way as hydraulic power is carried through a pipe. In Poynting's view the energy spreads out from the glass between the coatings of the jar and then converges sideways into the wire, where it is converted into heat. He showed that there was a general law for the transfer of energy, according to which it moves at any point perpendicularly to the planes containing the direction of the electric and magnetic forces, and the amount crossing unit area per second is equal to the product of these forces multiplied by the sine of the angle between them and divided by 4 π. The line which represents in direction and magnitude the flow of energy at any point is now known as the ‘Poynting vector’ and is of fundamental importance in electromagnetic questions.

Poynting made important advances in our knowledge of the pressure of light. He established the existence of the tangential force produced when light is reflected from a surface at which there is some absorption, and the existence of a torque when light passes through a prism, and succeeded in demonstrating the recoil from light of a surface giving out radiation. These experiments, in which he was associated with William Henry Barlow [q.v.], are a good example of Poynting's skill in devising methods and apparatus. He had exceptional mechanical instincts and an excellent knowledge of the capabilities of instruments. The result was that the apparatus which he designed was always simple and effective.

Throughout his life Poynting was engaged on researches connected with gravitation. His first piece of experimental work was a determination of the density of the earth, using, instead of a torsion balance, a balance of the ordinary type. He also investigated the question whether the gravitational attraction between two crystals depends on the orientation of their axes, and whether this attraction is affected by temperature. He further made important contributions to the theory of the change of state in matter. He took great interest in the philosophical basis of physics, chose this as the subject of his presidential address to Section A of the British Association in 1899, and expressed views which, though now common, were then different from those accepted by the majority of the physicists of this country.

Poynting's Collected Scientific Papers were published by the Cambridge University Press in 1920. In addition to these he wrote On the Mean Density of the Earth (Adams prize essay, 1893), The Pressure of Light (1910), The Earth (1913), and, in conjunction with (Sir) J. J. Thomson, a series of textbooks on physics. Poynting was very successful as a teacher, and his sound judgement and conspicuous fairness and courtesy were of great service to the university of Birmingham. He became dean of the science faculty when Mason College was made the university of Birmingham, and held the office for twelve years. He died at Birmingham 30 March 1914.

Poynting married in 1880 Maria Adney, daughter of the Rev. J. Cropper, Unitarian minister, of Stand, near Manchester. They had one son and two daughters.

[Personal knowledge.]

J. J. T.