Clock types · Special mechanisms

Anniversary clock (400-day clock)

A clock that runs for more than a year on one winding, thanks to a slowly twisting torsion pendulum, usually displayed under a glass dome.

By David Ray · clocks.fyi

What an anniversary clock is

An anniversary clock is a mechanical clock that only needs winding about once a year. The name reflects that habit: according to the NAWCC's National Watch and Clock Museum, the idea was to wind it on the anniversary of the last winding, and the clocks became a traditional gift for weddings and other yearly celebrations.

Collectors also call them 400-day clocks, after the running time, or torsion pendulum clocks, after the mechanism that makes the long run possible. Most stand under a glass dome, with a dial on top of a small pillar or frame and a weighted disc or set of balls slowly turning beneath it.

That long running time is the whole appeal. A standard household mechanical clock needs a key every week or so, while an anniversary clock can sit on a shelf for a year and keep going, which made it an easy, low-maintenance gift.

How the torsion pendulum works

A normal pendulum swings from side to side. A torsion pendulum instead rotates, turning a little way in one direction and then back again, while hanging from a long, thin suspension spring. Each turn twists the spring, and the spring's urge to untwist sends the pendulum back the other way.

This motion uses very little energy. The NAWCC museum points out that most mechanical clocks run about eight days per winding, while the torsion pendulum lets an anniversary clock keep going for more than a year between windings.

Anton Harder's 1882 U.S. patent describes the key parts. A horizontal disc hangs from a steel torsion spring, and a forked lever clamped to the spring connects to the anchor of the escapement. As the spring twists back and forth, the lever rocks the anchor, releasing the escape wheel a tooth at a time and receiving a small push to keep the pendulum going.

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History

The idea is older than the clocks most people know. The NAWCC museum credits Philadelphia-born Robert Leslie with inventing the torsion pendulum design, for which he received a British patent in 1793 while working in London. Aaron Dodd Crane received the first U.S. patent for a torsion clock design in 1841.

Crane, who worked in Newark, New Jersey, kept refining the idea. His U.S. patent No. 12,196 of January 9, 1855, describes a torsion pendulum with two or more weights on adjustable arms that move outward or inward like the governor balls of a steam engine. When cold stiffened the suspension spring and the pendulum tried to speed up, the weights moved outward and slowed it again, keeping each turn the same length.

The familiar domed anniversary clock grew out of the work of Anton Harder of Ransen, then in Prussia. His U.S. patent for a torsion-pendulum clock, No. 269,052, was filed on July 18, 1882, and issued on December 12, 1882. It notes earlier grants in Austria-Hungary dated August 17, 1880, and mentions the German Empire without giving a date. Some popular histories date Harder's first German patent to 1879, but that year does not appear in the U.S. patent, so treat it with caution.

Harder's patent frames the idea in practical terms. It covers a clock whose pendulum is arranged horizontally and rotates alternately in each direction, hanging from a steel torsion spring, with the disc, a slotted stud, clamps and a lever tying the spring to the anchor and escape wheel.

Germany's Black Forest became an early home of these clocks. The Deutsches Uhrenmuseum in Furtwangen holds a torsion pendulum clock of about 1885 made under the Harder patent and associated with the Jahresuhren-Fabrik, or year clock factory, in Triberg, with August Schatz and Sons listed as maker.

Museum examples

The Deutsches Uhrenmuseum's Harder-patent clock is a table clock about 27 cm high and 18 cm across, kept in the museum's collection of year clocks.

The Science Museum Group in London owns a German 400-day anniversary clock made between 1898 and 1915, possibly by the Jahresuhrenfabrik, though the museum notes that attribution is uncertain. It has a brass base, a disc pendulum, a six-pillared gallery and a circular glass shade, and it came with two winding keys. The clock alone is about 247 mm tall and weighs about 1.05 kg, rising to about 310 mm with its dome.

Both clocks show how settled the form became early on: a compact movement under a dial, a hanging disc pendulum below it and a protective glass cover over everything. The same basic layout is what most people picture today when they hear the words anniversary clock.

How to recognize one

Watch the pendulum. If the weight below the dial turns slowly to one side and then back, instead of swinging left and right, you are looking at a torsion pendulum. On the Harder-patent clocks and the Science Museum example, that weight is a disc.

Look for a long, very thin wire or ribbon running from the movement down to the pendulum. This suspension spring is delicate, and it is the part that does the timekeeping work.

Most anniversary clocks stand under a glass dome or shade on a round base, which keeps drafts and dust away from the slow-moving pendulum and its fine suspension spring.

Check how often it needs winding. A genuine anniversary clock is designed to run for a year or more on one winding, so an owner who winds it once a year, perhaps on a wedding anniversary, is using it exactly as intended.

See also: help when a mechanical clock refuses to keep running on OnlineClock.app.

Questions people ask

Why is it called an anniversary clock?

Because it can run for more than a year on one winding, so it could be wound once a year on an anniversary. It also became a traditional wedding and anniversary gift.

Who invented the 400-day clock?

Several people contributed. Robert Leslie patented the torsion pendulum in Britain in 1793, Aaron Dodd Crane received a U.S. torsion clock patent in 1841 and an improved one in 1855, and Anton Harder patented the design behind the familiar German domed clocks in Austria-Hungary in 1880 and the United States in 1882.

Why does the pendulum turn instead of swinging?

It is a torsion pendulum. It hangs from a thin spring that twists and untwists, which uses far less energy than a swinging pendulum and allows the clock to run for about a year.

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