Swiss 18K yellow gold lever with Guillaume balance Circa 1916

Gold hunting cased pocket watch
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An extremely fine and rare Swiss, 18K yellow gold hunting cased, keyless, lever chronometer with Guillaume balance, circa 1916. Bulletin 1ère Classe with honorable Mention Vacheron & Constantin, Genève. Chronomètre Bulletin de 1ere Classe aux Observatoires de Geneve et Teddington. Submitted for trial at the Geneva Observatory in 1920, regulated by A. Favre-Rochat. Accompanied by copies of the Geneva Observatory rating details.
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A solid, polished case, the interior of the back with engraved dedication. Hinged gold cuvette with inscription. White enamel dial with dauphine numerals, outer minute track, subsidiary seconds dial. Gold spade hands. A rhodium-plated movement, 20 jewels, wolf’s tooth winding, counterpoised and calibrated straight-line lever escapement, animal brass Guillaume balance, gold meantime and temperature adjustment screws, blued steel Breguet balance spring, swanneck micrometre regulator. Dial, case and movement signed.
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Diameter: 51 mm
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Signed: Vacheron & Constantin  Genève
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The watch was regulated for the Geneva Observatory trials by A. Favre-Rochat and obtained 723 points out of a possible 1000. This watch came 18th overall out of the entire 43 classes on trial. The Guillaume Balance, Anibal Acier au nickel pour balanciers, an alloy invented by Dr Charles Edouard Guillaume, exhibits unusual properties, both in terms of thermal expansion and in changes in elasticity.
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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 is 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. In 1899, Guillaume noted that steel with an addition of 44.4% nickel had a negative square coefficient of thermal expansion. Anibal, combined with brass in bimetallic laminae, makes expansion close to quadratic. Balances with bimetallic rims made of Anibal and brass are usually called Guillaume balances. When combined with special balance springs as in the present watch, they exhibit remarkable temperature stability, on occasion not exceeding 1/50 second per day at 1oC.