Rare and large English silver pocket chronometer Circa 1797

Silver important pocket watch by John Arnold
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A very fine and extremely rare and important English silver deck chronometer of the Best Kind with spring detent escapement and Arnold’s O-Z balance, circa 1797. This is the only original pocket chronometer by John Arnold with O-Z balance known to exist – in its original mahogany fitted deck box.
Silver consular polished and à goutte case with case maker’s punch mark TH (Thomas Hardy), turning pendant, and large lateral hinge. Numbered and signed enamel dial with Roman radial hours, auxiliary seconds, and blued spade hands. Full plate movement, fire gilt, frosted, signed, and keywind; with chain/fusee with Harrisons maintaining power, moulded cylindrical pillars, steel escape wheel, Arnold’s steel and brass O-Z balance with adjustment screws and adjustment nuts, free-sprung blued helical balance spring, very fine florally engraved and pierced balance cock, chatoned jewels on the chronometer, seconds and intermediate wheel, and chatoned diamond endstone on balance.
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Diameter: 73 mm
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Weight: 275 g
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Signed: John Arnold, London
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John Arnold produced about 94 pocket chronometers “of the Best Kind” in his life, and 37 still exist today. Around a quarter of those have been modified to a greater or lesser extent in that they have no Arnold escapement or balance, no original dial or case, etc. No. 20/63 is an original piece demonstrating Arnold’s tireless experimenting and tenacious search for greater precision.

The most interesting feature of the watch is certainly its type of balance. The engraving on the movement “No. 20/63” indicates that it was originally a “double S” balance. When Arnold revised his pocket chronometers, he often fitted them with a higher quality balance because he aimed for better temperature compensation. He experimented with the “double T” and the “double S” balance. Eventually, he developed the “OZ” balance – a standard long before Arnold finally created the so-called “Z” balance. No. 63, however, features another even more advanced balance type – the “ZO” balance. This balance consists of a bimetal Z-shaped piece connected to a smaller O-shaped brass ring with three screwed-on weights.

Interestingly, this balance in an Arnold pocket chronometer has always been attributed to his son Roger; by Arnold himself, there are only very few of these movements. This makes no. 63 is the only pocket chronometer by J. Arnold with this type of balance.