German silver lever chronometer with power reserve Circa 1942

Silver open faced deck watch by A. Lange & Söhne
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A fine German, keyless, silver, deck and pocket lever chronometer with power reserve indication and Guillaume balance, circa 1942. A polished case and back cover engraved with the watch issue number. Hinged silver steel cuvette. Brushed silvered dial with painted black Arabic numerals, outer minute track, large subsidiary seconds at 3, power reserve indication at 9, and blued steel spade hands. A frosted gilt movement, three-quarter plate, 17-jewels, straight-line lever escapement with lateral banking extension on the entry pallet, Anibal brass Guillaume balance, blued steel Breguet balance spring with patent stud fastening, swan-neck micrometre adjustment to the regulator. Case, cuvette and movement signed.
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Diameter: 60 mm
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Weight: 171 g
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Signed: A. Lange & Söhne  Glashütte/Sa.
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This model of deck watch was made for the Kriegsmarine Observation Service.
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Guillaume Balance: In 1899, Dr Charles Edouard 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 a bimetallic lamina, makes its expansion close to quadratic. Balances with bimetallic rims made of Anibal (Anibal, an alloy invented by Guillaume, stands for (Acier au Nickel pour BaLanciers) and brass are usually called Guillaume balances, or, as their inventor called them, integral balances. Combined with special balance springs, they exhibit remarkable temperature stability, occasionally not exceeding 1/50 second per day at 1oC.