Lloyd, Humphrey

(b. Dublin, Ireland, 16 April 1800; d. Dublin, 17 January 1881)

physics.

An active member of the Church of Ireland and provost of Trinity College, Dublin, Lloyd was known among physicists as an expert in optics and a leader in the British program to map the earth’s magnetic field. His career closely paralleled that of his father, the Reverend Bartholomew Lloyd, who was professor of mathematics, natural philosophy, Greek, and divinity at Trinity College, as well as provost of the college, and president of the Royal Irish Academy. Humphrey displayed his abilities while young, placing first on the entrance examination for Trinity and, upon his graduation in 1819, receiving a gold medal for excellence in science. He became a member of the faculty and rose through the ranks: junior fellow (1824), professor of natural and experimental science (1831), senior fellow (1843), vice provost (1862), and provost (1867). During this period he received his Master of Arts and Doctor of Divinity. He was president of the Royal Irish Academy from 1846 to 1851, and a member of the Royal Societies of London and Edinburgh and of the British Association for the Advancement of Science. At the age of forty he married Dorothea Bulwer, the daughter of a clergyman.

Lloyd’s first important original research was in optics. In 1832 W. R. Hamilton deduced from Fresnel’s equations for double refraction in a biaxial crystal the consequence that under certain conditions a ray of light within the crystal would split into an infinite number of rays forming a conic surface. Hamilton asked Lloyd to investigate this “conical refraction” experimentally, and Lloyd succeeded in giving a demonstration of the phenomenon and finding the direction of polarization of the rays making up the luminous cone. This achievement gave new support to the wave theory of light and established Lloyd’s reputation as an expert in optics.

In 1834, while examining Fresnel’s classic experiment in which an interference pattern is produced by light from a single source reflected by two mirrors, Lloyd discovered that a similar effect could be produced by using a single reflecting surface. Arranging a light source, mirror, and screen as shown in Figure 1, he caused reflected light to interfere with direct light from the source in a limited area on the screen. The location of the bands of this pattern could be deduced by imagining they were caused by a second source of light, located at the mirror image of the first and producing an interference pattern. This treatment enabled Lloyd to establish two important properties of reflection: first, that the intensity of light reflected at a 90° incidence is equal to that of direct light (as Fresnel had predicted), and second, that a half-wavelength phase acceleration takes place upon reflection by a higher-density medium.

Lloyd’s other contributions to optics include a report to the British Association on the state of the subject in 1833, two textbooks expounding the wave theory, and a study of reflection and refraction by thin plates. The bulk of his remaining scientific work was devoted to terrestrial magnetism.

In 1831-1832 the British Association proposed that a series of observations of the intensity of the earth’s magnetic field be made in various parts of the kingdom. It appointed a committee to coordinate this activity and Lloyd was made a member. Shortly thereafter he undertook a series of magnetic observations in Ireland with Sir Edward Sabine. During the course of this project Lloyd made measurements in twenty-four areas of his native land and invented a new technique for the simultaneous determination of magnetic inclination and intensity.

By 1838 the discovery of temporal variations in the earth’s magnetic field created the need for a set of permanent observatories capable of making simultaneous measurements in different parts of the world. The British Association, with government financial support, undertook the construction of these stations and assigned Lloyd the important job of drawing up instructions for the observers and teaching the officers in charge the use of the instruments. Lloyd’s own observatory at Trinity College, which had been constructed under his supervision in 1837, served as a model for the other stations and received the results of their observations.

Lloyd’s contributions to the understanding of terrestrial magnetism did not end with his role in setting up the British magnetic survey. He remained active in the field throughout his life and succeeded in demonstrating the existence of currents of electricity in the earth’s crust and in calculating their effect on the daily variation in the magnetic field.

BIBLIOGRAPHY

I. Original Works. Lloyd’s articles include “On the Phenomena Presented by Light in Its Passage Along the Axes of Biaxial Crystals,” in Philosophical Magazine,2 (1833), 112-120, 207-210; “Report on the Progress and Present State of Physical Optics,” in British Association Reports (1834), 295-413; “An Attempt to Facilitate Observations of Terrestrial Magnetism,” in Transactions of the Royal Irish Academy,17 (1837), 159-170; “Further Developments of a Method …,” ibid., 449-460; “On a New Case of Interference of the Rays of Light,“ibid., 171-178; “On the Light Reflected and Transmitted by Thin Plates,” ibid.,24, pt. 1 (1860), 3-15; and “On Earth-Currents, and Their Connexion With the Phenomena of Terrestrial Magnetism,” in Philosophical Magazine,22 (1861), 437-442. His most important scientific papers are reprinted in his Miscellaneous Papers Connected With Physical Science (London, 1877), and his other published works are listed in Dictionary of National Biography, XI, p. 1304. His textbooks are The Elements of Optics (Dublin, 1849) and Elementary Treatise on the Wave Theory of Light(Dublin, 1857).

II. Secondary Literature. The best biographical material on Lloyd is in the Proceedings of the Royal Society,31 (1881), 23. A bibliography of his scientific papers is in Royal Society, Catalogue of Scientific Papers, IV, 62-63.

Eugene Frankel