Torsion Pendulum
American Watchmaker and Jeweler (1892)
A torsion pendulum is one that depends for its vibrations upon the twisting and untwisting of an elastic suspension. The path of a torsion pendulum is unlike that of other pendulums, as it does not swing from right to left, but simply revolves horizontally, the suspension acting as an axis. The action of a torsion pendulum is very simple; after turning the bob or weight and releasing it, the elasticity of the wire or other suspension returns it to the point of rest and the momentum of the weight carries it forward, twisting the suspension in the opposite direction, until the weight reaches a point where the momentum of the weight is overbalanced by the resistance of the suspension, when the suspension again untwists, turning the bob in the opposite direction. These oscillation, which, within certain limits are very nearly equal, continue until the force originally applied is exhausted in friction. The application of the torsion pendulum to clocks has been successfully accomplished. By its use either of two results may be secured; the time of running may be prolonged in proportion as the period of the torsion pendulum is longer than that of an oscillating one, or the number of gear wheels required in the clock may be greatly reduced. Ordinary clocks have been constructed on this principle that would run a year with a single winding, while others have been constructed that would run a much longer period. Fig.249 illustrates a simple torsion pendulum that can easily be constructed for experimental purposes. It has parallel suspension wires inch apart and 5 feet long, made from No. 30 brass spring wire. The bob is formed of a disc of metal, with a series of split lead balls pinched down upon its edge. It has a double loop fixed at the center for fastening the suspension wires. The diameter of the disc is 4 inches and the total weight of bob 1 1/2 pounds.
What Has Changed in the Last Century
The physics described by the American Watchmaker and Jeweler in 1892 is exactly right and still taught in this form; the torsion pendulum's period depends on the moment of inertia of the bob and the torsional stiffness of the suspension, and it remains a working laboratory instrument. What the 1892 writer could not foresee was how far the long-period clock idea would run: the torsion pendulum became the heart of the 400-day "anniversary" clock under its glass dome, and of Jean-Leon Reutter's Atmos clock, designed in the late 1920s and made by Jaeger-LeCoultre, which needs no winding at all because small changes in room temperature expand a sealed gas capsule that rewinds the spring. Both were largely swept aside as timekeepers by cheap quartz movements from the 1970s onward, surviving as ornaments. In physics the same device went the other way and got far more precise: torsion balances of this type are still the standard tool for measuring the gravitational constant and for testing whether gravity acts equally on all materials, and suspension fibres behaving as torsion pendulums are part of the mirror suspensions in gravitational-wave detectors.
Written for this edition, 2026.