Large Swiss silver deck and pocket lever chronometer Circa 1944

Gold lever watch with centre-seconds
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A fine Swiss, large, silver, keyless centre-seconds deck and pocket lever chronometer with Guillaume balance, circa 1944. Made for the British Hydrographic Service.
A polished case, silver, hinged cuvette. Silver, brushed dial, Arabic numerals, outer minute divisions. Blued steel spade hands. A gilt brass movement, Geneva striping decoration, 21 jewels, straight-line calibrated lever escapement, Anibal-brass Guillaume balance with gold timing screws, blued steel Breguet balance spring, swan-neck micrometric regulator. Dial, case and movement signed, case back engraved H.S.- 2,55028.
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Diameter: 60 mm
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Signed: Vacheron & Constantin  Genève
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Anibal An alloy invented by the Nobel prize winner Dr Charles Edouard Guillaume, 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 a change in the elasticity of a balance spring is approximately a quadratic function. Thus, it equals zero at only two temperatures, causing a secondary error.
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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 the 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.