Figure 10.—Henry Kater (1777-1835), English army officer and physicist. His scientific career began during his military service in India, where he assisted in the “great trigonometrical survey.” Returned to England because of bad health, and retired in 1814, he pioneered (1818) in the development of the convertible pendulum as an alternative to the approximation of the “simple” pendulum for the measurement of the “seconds pendulum.” Kater’s convertible pendulum and the invariable pendulum introduced by him in 1819 were the basis of English pendulum work. (Photo courtesy National Portrait Gallery, London.)
Figure 11.—The attempt to approximate the simple (theoretical) pendulum in gravity experiments ended in 1817-18 when Henry Kater invented the compound convertible pendulum, from which the equivalent simple pendulum could be obtained according to the method of Huygens (see text, [p. 314]). Developed in connection with a project to fix the standard of English measure, Kater’s pendulum was called "compound" because it was a solid bar rather than the fine wire or string with which earlier experimenters had tried to approximate a "weightless" rod. It was called convertible because it is alternately swung from the two knife edges (a and b) at opposite ends. The weights (f and g) are adjusted so that the period of the pendulum is the same from either knife edge. The distance between the two knife edges is then equal to the length of the equivalent simple pendulum.
The systematic survey of the gravity field of the earth was given a great impetus by the contributions of Capt. Henry Kater, F.R.S. In 1817, he designed, constructed, and applied a convertible compound pendulum for the absolute determination of gravity at the house of Henry Browne, F.R.S., in Portland Place, London. [24] Kater’s convertible pendulum ([fig. 11]) consisted of a brass rod to which were attached a flat circular bob of brass and two adjustable weights, the smaller of which was adjusted by a screw. The convertibility of the pendulum was constituted by the provision of two knife edges turned inwards on opposite sides of the center of gravity. The pendulum was swung on each knife edge, and the adjustable weights were moved until the times of swing were the same about each knife edge. When the times were judged to be the same, the distance between the knife edges was inferred to be the length of the equivalent simple pendulum, in accordance with Huygens’ theorem on conjugate points of a compound pendulum. Kater determined the time of swing by the method of coincidences ([fig. 12]). He corrected for the buoyancy of the air. The final value of the length of the seconds pendulum at Browne’s house in London, reduced to sea level, was determined to be 39.13929 inches.
The convertible compound pendulum had been conceived prior to its realization by Kater. In 1792, on the occasion of the proposal in Paris to establish the standard of length as the length of the seconds pendulum, Baron de Prony had proposed the employment of a compound pendulum with three axes of oscillation. [25] In 1800, he proposed the convertible compound pendulum with knife edges about which the pendulum could complete swings in equal times. De Prony’s proposals were not accepted and his papers remained unpublished until 1889, at which time they were discovered by Defforges. The French decision was to experiment with the ball pendulum, and the determination of the length of the seconds pendulum was carried out by Borda and Cassini by methods previously described. Bohnenberger in his Astronomie (1811), [26] made the proposal to employ a convertible pendulum for the absolute determination of gravity; thus, he has received credit for priority in publication. Capt. Kater independently conceived of the convertible pendulum and was the first to design, construct, and swing one.
After his observations with the convertible pendulum, Capt. Kater designed an invariable compound pendulum with a single knife edge but otherwise similar in external form to the convertible pendulum [27] ([fig. 13]). Thirteen of these Kater invariable pendulums have been reported as constructed and swung at stations throughout the world. [28] Kater himself swung an invariable pendulum at a station in London and at various other stations in the British Isles. Capt. Edward Sabine, between 1820 and 1825, made voyages and swung Kater invariable pendulums at stations from the West Indies to Greenland and Spitzbergen. [29] In 1820, Kater swung a Kater invariable pendulum at London and then sent it to Goldingham, who swung it in 1821 at Madras, India. [30] Also in 1820, Kater supplied an invariable pendulum to Hall, who swung it at London and then made observations near the equator and in the Southern Hemisphere, and at London again in 1823. [31] The same pendulum, after its knives were reground, was delivered to Adm. Lütke of Russia, who observed gravity with it on a trip around the world between 1826 and 1829. [32]
