KATER, HENRY (1777-1835), English physicist of German descent, was born at Bristol on the 16th of April 1777. At first he purposed to study law; but this he abandoned on his father's death in 1794, and entered the army, obtaining a commission in the xath regiment of foot, then stationed in India, where he rendered valuable assistance in the great trigonometrical survey. Failing health obliged him to return to England; and in 1808, being then a lieutenant, he entered on a distinguished student career in the senior department of the Royal Military College at Sandhurst. Shortly after he was promoted to the rank of captain. In 1814 he retired on half-pay, and devoted the. remainder of his life to scientific research. He died at London on the 26th of April 1835.
His first important contribution to scientific knowledge was the comparison of the merits of the Cassegrainian and Gregorian telescopes, from which (Phil. Trans., 1813 and 1814) he deduced that the illuminating power of the former exceeded that of the latter in the proportion of 5 : 2. This inferiority of the Gregorian he explained as being probably due to the mutual interference of the rays as they crossed at the principal focus before reflection at the second mirror. His most valuable work was the determination of the length of the second's pendulum, first at London and subsequently at various stations throughout the country (Phil. Trans., 1818, 1819). In these researches he skilfully took advantage of the well-known property of reciprocity between the centres of suspension and oscillation of an oscillating body, so as to determine experimentally the precise position of the centre of oscillation; the distance between these centres was then the length of the ideal simple pendulum having the same time of oscillation. As the inventor of the floating collimator, Kater rendered a great service to practical astronomy (Phil. Trans., 1825, 1828). He also published memoirs (Phil. Trans., 1821, 1831) on British standards of length and mass; and in 1832 he published an account of his labours in verifying the Russian standards of length. For his services to Russia in this respect he received in 1814 the decoration of the order of St. Anne; and the same year he was elected a fellow of the Royal Society.
His attention was also turned to the subject of compass needles, his Bakerian lecture " On the Best Kind of Steel and Form for a Compass Needle" (Phil. Trans., 1821) containing the results of many experiments. The treatise on " Mechanics" in Lardner's Cyclopaedia was partly written by him; and his interest in more purely astronomical questions was evidenced by two communications to the Astronomical Society's Memoirs for 1831-1833 the one on an observation of Saturn's outer ring, the other on a method of determining longitude by means of lunar eclipses.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)