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The 400-day clock is characterised by the fitting of a torsion pendulum. Instead of swinging from side to side, as in conventional clocks, the torsion pendulum rotates on the end of a long thin suspension spring, and is in fact more akin to a balance wheel than a pendulum.
Although torsion clocks appeared as early as 1841, made to the patent of Aaron Crane of New Jersey, they only became really popular from about 1881, when production started in the Black Forest region of Germany. August Schatz founded the Wintermantel Company, which later (1884) became the Jahresuhrenfabrik Company. This company produced torsion clocks to the patent specification of Anton Harder, which described the torsion clock which is substantially the same as that which has been produced so succesfully ever since.
In 1887, the original patent lapsed, and several other German manufacturers started to produce torsion clocks. As well as Jahresuhrenfabrik, the most well known companies included Gustav Becker, Kieninger & Obergfell (Kundo), Lenzkirch, Kienzle and Junghans. Most of these produced clocks of similar style and appearance, usually housed under a glass dome.
The earliest examples of these clocks used a flat brass disc for the rotating pendulum bob, with two smaller adjustable discs on top of this to provide timing adjustments. Later models adopted four (sometimes three) brass balls, with greater or lesser degrees of ornamentation. The clocks are all amazingly efficient - in spite of the relatively small mainspring, they will run for upwards of a year. This long running time is achieved in two ways - firstly, the long period of the torsion pendulum, usually 8 turns per minute, and secondly, by the introduction of additional 'intermediate' wheels and pinions between the mainspring barrel and the remainder of the movement.
The escapement is usually a modified form of the Graham 'dead beat', operated by a small fork and lever at the top end of the suspension wire. This wire is extremely fragile, and its dimensions are very critical if accurate timekeeping is to be achieved. The wire is in fact rectangular in cross section, and over the years many different materials have been used. The earliest wires were of hardened and tempered steel, but following problems with rusting, this was later replaced with bronze. Still later, low expansion alloys such as Invar or Ni-span were introduced to provide temperature compensation. Because of these variations, when replacing a suspension wire, it is not sufficient to measure the physical dimensions of the old wire - the material has to correctly selected as well.
400 day clocks achieved a high degree of popularity in the United States, particularly during the last war, when returning ex-servicemen acquired them as souvenirs of their time spent in Europe. The clocks are still being produced, although much use is now being made of plastics for the component parts, and there are in fact many examples where the torsion pendulum is merely a dummy, attached to a modern quartz electrical movement.
From a collectors point of view, the most desirable clocks are those produced before 1914, particularly those from the factories of Jahresuhenfabrik and Gustav Becker.
Charles Terwilliger, The 400-day Clock repair Guide, Horolovar 1984, ISBN 0 916316 04 1
A & R Shenton, Collectable Clocks, Antique Collectors Club 1985, ISBN 1 85149 195 3
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