Pendulum

Pendulum, device consisting of an object suspended from a fixed point that swings back and forth under the influence of gravity. Pendulums are used in several kinds of mechanical devices, such as certain types of clocks.

In the most basic type of pendulum, the simple pendulum, which oscillates back and forth in a single plane, all the mass of the device can be considered to reside entirely in the suspended object. The motion of pendulums in clocks closely approximates the motion of a simple pendulum. A spherical pendulum, however, is not confined to a single plane, and therefore has a much more complicated motion than that of a simple pendulum.

The principle of the pendulum was discovered by Italian physicist and astronomer Galileo, who established that the period for the back-and-forth oscillation of a pendulum of a given length remains the same, no matter how large its arc, or amplitude. (However, if the amplitude is too large, the period of the pendulum is dependent on the amplitude.) Galileo noted the possible applications in timekeeping of this phenomenon, called isochronism. Because of the role played by gravity, however, the period of a pendulum is related to geographical location, because the strength of gravity varies as a function of latitude and elevation. For example, this period will be greater on a mountain than at sea level. Thus, the pendulum can be used to determine accurately the local acceleration of gravity.

The simple pendulum, used for timekeeping, is accurate as a regulator, if the proper length of the rod is preserved. It was found, however, that in winter clocks went too fast and at midsummer too slow, because low temperatures shortened the metallic rod and heat lengthened it. A refinement was made to ensure uniform length and accurate timekeeping by the use of compensation pendulums, the main types being the mercury pendulum and the gridiron pendulum. The mercury pendulum carries a glass cylinder almost full of mercury. When the pendulum expands downwards because of heat, the change is counterbalanced by the upward expansion of the mercury in the cylinder. The gridiron pendulum is composed of a series of upright metal bars, usually of steel and copper, having different compositions and therefore different coefficients of thermal expansion. If the relative lengths of these bars are carefully adjusted, changes of temperature do not affect the pendulum's timekeeping.

Various types of scientific instruments incorporate the bifilar pendulum, the Foucault pendulum, and the torsion pendulum. Bifilar pendulums, which employ two strings or wires, have been used to record the irregular rotation of the Earth as well as to detect earthquakes. The Foucault pendulum is used to demonstrate the rotation of the Earth. Named after the French physicist Jean Bernard Leon Foucault, it consists of a heavy bob suspended on a long wire; Foucault used a 28-kg (62-lb) bob attached to a 67-m (220-ft) wire. After the pendulum is set in motion so that it swings back and forth in a single plane, the rotation of the Earth causes the orientation of the back-and-forth motions of the pendulum to slowly rotate with respect to the ground underneath the pendulum. The effect is most pronounced at the North Pole, where the pendulum rotates once every 24 hours. The rate of the pendulum's rotation with respect to the ground decreases with latitude; at the equator the pendulum does not rotate at all.

A torsion pendulum consists of a wire or similar fibre. The pendulum oscillates by repeatedly twisting and untwisting about the axis through the centre of the wire. Though it is not strictly a pendulum, as it does not oscillate because of the force of gravity, the mathematical formulae that describe torsion pendulum motion are similar to those describing simple pendulum motion. (See Torsion; Torsion Balance).

See Equilibrium.