Collier's New Encyclopedia (1921)/Pendulum

PENDULUM, 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 of the arc 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 first escapement. Henry de Wyck (1364), Harris (1641), and Huyghens (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 chord of that arc the axis of oscillation, and the 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; and at Spitzbergen, 39.21469 inches.