Swiss rose gold Adjusted Extra lever chronometer Circa 1898

Rose gold open-faced keyless lever with Guillaume balance by Patek Philippe
–
A very fine and rare Swiss 18-carat rose gold open-faced Adjusted Extra keyless lever chronometer pocket watch with Guillaume balance, circa 1898. Sold on June 12, 1900. Nickel-silver movement with 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 inner and outer terminal curve, diamond endstone, swan-neck micrometer regulator with index. Gold hinged cuvette. White enamel dial with Breguet numerals, outer minute track, subsidiary seconds dial, and original blued steel spade hands. Plain case, engraved monogram to the back. Dial, case, and movement signed, case and movement numbered. Comes with the Extract from the Archives and Geneva Observatory Rating Certificate Grade A obtained on November 11, 1899.
–
Diameter: 51 mm
–
Signed: Patek, Philippe & Cie Genève
–
When destined for Observatory Timing Contests, the movements of Patek Philippe watches, like those of other makers, were 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 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 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.