Anniversary clock

A torsion pendulum clock, or torsion clock (also known as 400-day or anniversary clock), is a mechanical clock which keeps time with a mechanism called a torsion pendulum. This is a weighted disk or wheel, often a decorative wheel with 3 or 4 chrome balls on ornate spokes, suspended by a thin wire or ribbon called a torsion spring (also known as "suspension spring"). The torsion pendulum rotates about the vertical axis of the wire, twisting it, instead of swinging like an ordinary pendulum. The force of the twisting torsion spring reverses the direction of rotation, so the torsion pendulum oscillates slowly, clockwise and counterclockwise. The clock's gears apply a pulse of torque to the top of the torsion spring with each rotation to keep the wheel going. The wheel and torsion spring function similarly to a watch's balance wheel and hairspring, as a harmonic oscillator to control the rate of the clock's hands.

Torsion clocks are usually delicate, ornamental, spring-wound mantel clocks. The polished clock mechanism is exposed under a glass case or dome, to allow people to watch the torsion pendulum turn. Clocks of this style, first made by Anton Harder around 1880, are also known as 400-day or anniversary clocks, because many can run for an entire year on a single winding. This does not mean they will keep accurate time the whole year. It's best to wind the clock once a month. But some models will run up to 1000 days on a single winding.

Torsion clocks are capable of running much longer between windings than clocks with an ordinary pendulum, because the torsion pendulum rotates slowly and takes little energy. However they are difficult to set up and are usually not as accurate as clocks with ordinary pendulums. One reason is that the oscillation period of the torsion pendulum changes with temperature due to temperature-dependent change in elasticity of the spring. The rate of the clock can be made faster or slower by an adjustment screw mechanism on the torsion pendulum that moves the weight balls in or out from the axis. The closer in the balls are, the smaller the moment of inertia of the torsion pendulum and the faster it will turn, like a spinning ice skater who pulls in her arms. This causes the clock to speed up.

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Wikipedia