ROWLAND, HENRY AUGUSTUS I_4_ ( g 8 ,--I901), Ameri can physicist, was born at Honesdale, Pa., on Nov. 27, 1848. He at as a civil engineer at Rensselaer Polytechnic nstitute at Troy in 187o, and two years later returned there as instructor in physics, becoming assistant professor in 1877. While Troy he made investigations on magnetic induction, permeability and distribution, which established fundamental results. In 1875 he was chosen to occupy the chair of physics in the newly-founded Johns Hopkins university, a position which he held until his death, at Baltimore, on April 16, 19or. Before beginning his work at Johns Hopkins he went to Europe, to visit the various physics research centres of the Continent, and to purchase laboratory apparatus. He studied under Helmholtz in Berlin, where he car ried out experiments proving that an electrostatic charge carried at a high rate of speed had the same magnetic action as an elec tric current. At Johns Hopkins he carried on a long series of experiments in which he computed the accepted value of the me chanical equivalent of heat, experiments which necessitated more careful thermometric and calorimetric methods than had ever been used before. Similar refined apparatus and technique en abled him to make a more nearly accurate determination of the value of the ohm, than had been calculated before. Becoming in
terested in the study of spectrum analysis, he realized the impor tance of securing more accurate diffraction gratings, and to this end constructed a dividing engine which allowed from 14,000 to 20,000 grating lines to be ruled to the inch on a plane surface of either glass or speculum metal. He next investigated the action of a grating ruled on a spherical concave surface, and, discovering the advantages, proceeded to rule them. These gratings came to be used in physics laboratories the world over, and the modern study of spectroscopy as an exact science dates from this work. With these gratings Rowland studied and photographically mapped the solar spectrum for the first time. He then undertook the sys tematic study of the arc-spectra of all the elements so far as possible, and published his results between 1895 and 1900. In his last years he became interested in alternating currents and their practical application to motors, measuring instruments, etc. He devised a system of multiplex telegraphy depending upon syn chronous motors which received a grand medal at the Paris Expo sition of 1900. He was the recipient of many honours, including the Rumford and Draper medals.
See H. A. Rowland, Physical Papers (1902), which contains a bibliography of his writings.