Unusual French 18k gold chronograph dress watch Circa 1900

Gold watch with special antimagnetic double case
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A very fine French, rare and unusual, 18K gold, keyless dress watch with chronograph, instantaneous 60-minute register and special antimagnetic double case, circa 1900.
The back is engraved with a monogram, an inner soft iron case forming an antimagnetic shield. White enamel dial with radial Roman numerals, inner red Arabic 13-24, outer minutes and fifths of a second track, outermost tachymeter scale, subsidiary seconds at 6 and 60-minute register at 12. Gold Spade hands. A gilt brass movement, 23 jewels, straight-line lever escapement, bimetallic compensation balance cut in the middle of the rim, blued steel Breguet balance spring, index regulator, chronograph with visible work, activated by a button in the winding-crown. Dial, cuvette and movement signed.
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Diameter: 51 mm
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Signed: L. Leroy & Cie.  Paris
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The case of the present watch is an early example of the type later used by manufacturers such as Rolex, Omega and IWC to solve the problem of stray magnetic fields magnetising the movements of a watch.