Swiss indirect centre seconds lever deck watch Circa 1945

Silver open faced keyless lever by Vacheron & Constantin
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A fine, large Swiss silver open faced keyless lever deck chronometer with indirect centre seconds, made in the 1940s. Gilt movement, 20 jewels, straight-line lever escapement, anibal brass Guillaume balance with gold temperature and meantime adjustment screws, blued steel Breguet balance spring, swan neck micrometer regulator. Hinged silver cuvette. Brushed silver dial with champlevé Arabic numerals, outer minute and seconds divisions and Arabic five minute/second numerals and original blued steel spade hands. Massive, polished case. Dial, case and movement signed, case numbered.
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Diameter: 60 mm
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Signed: Vacheron & Constantin Genève Swiss
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The Anibal brass Guillaume balance is invented by the Nobel prize winner Dr. Charles Edouard Guillaume, it exhibits unusual properties, both in terms of thermal expansion and in changes in elasticity. These properties are very different from those of two other famous alloys invented by Guillaume, Invar and Elinvar. Around 1900 Guillaume attempted to eliminate the so called middle temperature error caused by the fact that the change of rate in a timekeeper with a steel brass bimetallic balance is approximately a linear function of temperature, while the change of rate caused by change in elasticity of a balance spring is approximately a quadratic function. Thus, it equals zero at only two temperatures, causing secondary error. Countless attempts were made to eliminate middle temperature error, usually by means of auxiliary compensation devices. In 1899, Guillaume noticed that steel with an addition of 44.4% nickel had a negative square coefficient of thermal expansion. This alloy, combined with brass in bimetallic lamina, makes its expansion close to quadratic. Balances with bimetallic rims made of anibal and brass are usually called Guillaume balances, or, as their inventor called them, integral balances. When combined with special balance springs, they exhibit remarkable temperature stability, on occasion not exceeding 1/50 second per day at 1oC.