Editing Pendulum clock (section)

==Torsion pendulum==
{{main|Torsion pendulum clock}}

Also called torsion-spring pendulum, this is a wheel-like mass (most often four spheres on cross spokes) suspended from a vertical strip (ribbon) of spring steel, used as the regulating mechanism in [[torsion pendulum clock]]s. Rotation of the mass winds and unwinds the suspension spring, with the energy impulse applied to the top of the spring. The main advantage of this type of pendulum is its low energy use; with a period of 12–15 seconds, compared to the gravity swing pendulum's period of 0.5—2s, it is possible to make clocks that need to be wound only every 30 days, or even only once a year or more. Since the [[restoring force]] is provided by the [[Elasticity (physics)|elasticity]] of the spring, which varies with temperature, it is more affected by temperature changes than a gravity-swing pendulum. The most accurate torsion clocks use a spring of [[elinvar]] which has low temperature coefficient of elasticity.

A torsion pendulum clock requiring only annual winding is sometimes called a "'''400-Day clock"''' or "'''anniversary clock'''", sometimes given as a [[wedding]] gift. Torsion pendulums are also used in "perpetual" clocks which do not need winding, as their [[mainspring]] is kept wound by changes in atmospheric temperature and pressure with a bellows arrangement. The [[Atmos clock]], one example, uses a torsion pendulum with a long oscillation period of 60 seconds.