Swiss gold Adjusted Extra lever pocket chronometer Circa 1900

Gold open faced keyless lever with Guillaume balance by Patek Philippe
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A very fine and rare Swiss 18 carat gold open faced keyless lever pocket chronometer with Guillaume balance, circa 1900. Adjusted Extra, rhodium-plated movement, Geneva Striping decoration, 17 jewels, wolf’s tooth winding, counterpoised straight-line lever escapement, Anibal brass Guillaume balance with winged arms, gold temperature and platinum meantime adjustment screws, blued steel Breguet balance spring with outer terminal curve, swan-neck micrometre regulator with index, diamond endstone. Gold hinged cuvette. White enamel dial with Roman numerals, outer minute track and Arabic five minute numerals, subsidiary seconds dial and original blued steel spade hands. Plain case, engraved monogram to the back. Dial and movement signed, case and movement numbered. Comes with the Extract from the Archives.
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Diameter: 50 mm
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Signed: Patek, Philippe & Cie Genève
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The inscription ‘Adjusted Extra’ on the bridge of the movement refers to the high finishing of all the parts of the movement.
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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. 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 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 1o°C.