Swiss 18 carat rose gold open faced pocket watch Circa 1898

Gold keyless lever chronometer watch
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A very fine and rare Swiss, 18K rose gold keyless Adjusted Extra lever chronometer pocket watch with Guillaume balance, circa 1898. Geneva Observatory Rating Certificate Grade A obtained on November 11, 1899. Accompanied by the Extract from the Archives. Sold on June 12, 1900. A polished case, back cover with engraved monogram. Hinged gold cuvette. White enamel dial with Breguet numerals, outer minute track, and subsidiary seconds dial. Blued steel spade hands.
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A rhodium-plated movement, Geneva striping decoration, engraved twice with the movement number, 17 jewels, wolf’s tooth winding, straight-line counterpoised and calibrated lever escapement, Anibal-brass Guillaume balance with winged arms, gold temperature adjustment screws and platinum mean time screws, blued steel Breguet balance spring with the inner and outer terminal curve, diamond endstone, swan-neck micrometre regulator with index. Dial, case and movement signed.
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Diameter: 51 mm
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Signed: Patek, Philippe & Cie.  Genève
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When destined for Observatory Timing Contests, Patek Philippe watches – as did those of other makers – had their movements double-engraved with serial numbers. The inscription “Adjusted Extra” on the bridge of the movement refers to the high finishing of all the parts of the movement. The Guillaume Balance. Anibal Acier au nickel pours balanciers, an alloy invented by Dr Charles Edouard Guillaume, exhibits unusual properties, in thermal expansion and in changes in elasticity. 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 the 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.