Lot No. 540 (91st Auction)
John Roger Arnold, London, Inv. et Fecit, Movement No. 20/63, 73 mm, 275 g, circa 1797

An important pocket chronometer "of the Best Kind" with spring detent escapement and Arnold's ZO balance. This is the only original pocket chronometer by John Roger Arnold with ZO balance known to exist - in its original mahogany fitted deck box
Case: silver, "consular" case, polished, à goutte, case maker's punch mark "TH" (Thomas Hardy), turning pendant, large lateral hinge. Dial: enamel, diameter 63 mm, radial Roman hours, auxiliary seconds, numbered, signed, blued spade hands. Movm.: full plate movement, firegilt, frosted, signed, keywind, chain/fusee with Harrisons maintaining power, moulded cylindrical pillars, steel escape wheel, Arnold's steel and brass ZO balance with adjustment screws and adjustment nuts, freesprung blued helical balance spring, very fine florally engraved and pierced balance cock, chatoned jewels on chronometer, seconds and intermediate wheel, chatoned diamond endstone on balance.

John Arnold produced about 94 pocket chronometers "of the Best Kind" in his life and 37 of them still exist today. Around a quarter of those have been modified to a greater or lesser extent in that they have no Arnold excapement or balance, no original dial or nor original case etc..
No. 20/63 is an original piece and demonstrates Arnold’s tireless experimenting and his indefatigable 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 was aiming for better temperature compensation. He experimented with the "double T" and the "double S" balance and eventually developed the "OZ" balance - which was a standard for a long time 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 that is connected to a smaller O-shaped brass ring with three screwed-on weights. It is extremely interesting that this kind of balance in an Arnold pocket chronometer has so far always been attributed to his son Roger; by Arnold himself there are only very few of these movements.
This makes no. 63 the only pocket chronometer by J. Arnold with this type of balance.
Source: Heinrich Boerner: John Arnold, London, No. 20/63. In: "Klassik Uhren, Journal für Freunde klassischer Zeitmesser", Ulm 3/1999, p. 30ff.

This watch is also described and illustrated in:
- Vaudrey Mercer, "John Arnold and Son, Chronometer Makers 1762-1843, London 1972 and Supplement London 1975, p. 318.
- Hans Staeger, "100 Jahre Präzisionsuhren von John Arnold bis Arnold & Frodsham 1763-1862", Filderstadt 1997, p. 67.
- Heinrich Boerner: John Arnold, London, No. 20/63. In: "Klassik Uhren, Journal für Freunde klassischer Zeitmesser", Ulm 3/1999, p. 30ff.
Estimate  50,000 - 70,000 €
This is a lot of a former auction!

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