Pendulum
Collier's New Encyclopedia (1921)
in mechanics, a simple pendulum is a heavy particle suspended by a fine thread from a fixed point, about which it oscillates without friction. The time of its vibration is directly as the square root of the length, and inversely as the square root of the accelerating force of gravity. The length th of the are through which it vibrates does not affect the result. No simple pendulum can exist; all constructed by man are compound pendulums in which there gravitates, not a particle, but a heavy body called the bob, the law of friction of course operating. In horology, the ordinary pendulum is believed to have been the invention of Ebn Junius of the University of Cordova, about A. D. 1100, his companion, Gerbert (poisoned in 1102), making the de Wyck first escapement. Henry Henry (1641), and Huyghens (1364), Harris Harris (about 1657), applied it to clocks; Galilei, in 1581, having recommended a pendulous weight as a true measurer, and Sanitorius, in 1612, the combination of a pendulum with wheel work. Pendulums generally move in arcs of circles. In the cycloidal pendulum the rod of suspension describes the arc of a cycloid, and in the conical a cone. Heat lengthens, and cold contracts the rod of a pendulum, if it be of a single metal, as steel or iron. To neutralize these effects compensation pendulums are made; the gridiron pendulum having bars of iron and brass to work against each other, and the mercurial pendulum making the center of the oscillation of the bob uniform by the expansion and contraction of mercury inside. The curved line along which the bob of a pendulum moves is called the arc of vibration, the horizontal of that are the axis of oscillation, and the chord point around which the pendulum moves the point of suspension, or the center of motion. The length of a pendulum vibrating seconds is directly proportionate to the force of gravity at the place. One constructed to beat seconds at London (lat. of Greenwich Observatory, 51° 28' N.) at the sea-level must measure 39.13983 inches; at the equator, 39.02074 inches. celebrated Grecian
What Has Changed in the Last Century
The physics Collier's set out in 1921 -- period varying as the square root of the length, a seconds pendulum of about 39.14 inches at London -- is still taught in exactly those terms. What has gone is the pendulum's job: it was the world's best timekeeper when this was written, but the Shortt free-pendulum clocks of the 1920s were the last word in that line, overtaken by quartz oscillators from 1927 and by caesium atomic clocks from 1955. Since 1967 the second has been defined by a caesium atomic transition rather than by any swinging weight, and local gravity is now measured with falling-corner-cube and superconducting gravimeters and from satellites rather than by timing a pendulum. Collier's garbled history is also worth reading with care: the pendulum clock is credited to Christiaan Huygens in 1656-57, following Galileo's observation of isochronism and the design his son Vincenzo attempted in 1641.
Written for this edition, 2026.