- Museum number
SILVER-CASED CENTRE-SECONDS POCKET-CHRONOMETER WITH PIVOTED DETENT ESCAPEMENT WITH ALTERED BALANCE AND BARREL.
Full plate; now with going barrel.
Arnold pivoted detent escapement; Later Arnold 'double T' balance.
Later white enamel dial, hours I-XII, minutes sub-divided into 1/5th seconds. Gold hour hand and blued-steel centre seconds hand probably original, minute hand broken and mostly missing.
Later silver case with spurious hallmarks.
- Production date
Diameter: 47.10 millimetres (back plate)
Diameter: 65 millimetres (case)
Diameter: 54.60 millimetres (dial plate)
Diameter: 48.60 millimetres (front plate)
Height: 32.26 millimetres (case)
Thickness: 10 millimetres (movement)
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by John Arnold, c. 1773
Signature: On the back plate 'John Arnold, London Inv. et Fecit No. 3'.
Case: Later silver consular case, with fly springs and pendant lock to open the back, Fake hallmarks inside the back only, probably intended to represent those for London in 1812. Joints not fitted with the usual bearers. Diam. 65.0 mm, h. over glass 32.26 mm.
Dial and hands: Later enamel dial, unsigned on either side but with ss on the underside fired in, held by three feet and pins in crudely recessed sinks in the dial plate.
Centre seconds hand carried between the minute and hour hands. The minute hand broken off but was not original, being fitted on a squared tube which is itself adjusted on a square on the cannon pinion. The gold hour hand probably original, with the pipe removed from the boss, and soldered to a brass tube adjusted on the hour wheel pipe. The blued steel seconds hand also appears to be original.
Dial plate diam. 54.6 mm; front plate diam. 48.6 mm; back plate diam. 47.1 mm; frame h. 10.0 mm.
Frame: The dial plate made up from a disc and a ring hard soldered together and gilded on the visible parts, possibly using part of the original dial plate for the inner part. The movement hinge held by rivets on the dial plate. The case catch fitted on the inside of the front plate. A slot for a previous catch has been filled and gilded over, leaving a block standing proud on the dial side. This block later filed down somewhat, removing the layer of gilding on that part. Remainder of the frame conventional, with four pillars and pins, the brass parts gilded. There are various unused holes in the plates including some that were filled before gilding. The cock relieved to clear the new clickwork for the going barrel.
Fusee now changed to going barrel: The fusee was originally planted in the holes now occupied by the going barrel, the holes for the original barrel are no longer used. Setting-up-work was fitted under the dial, and marks around the holes indicate its presence there before the conversion. This is confirmed by two setting-up dots by the arbor pivot hole on the back plate. The fusee stop-work block removed by being broken off flush with the back plate. The original fusee did not have Harrison's main¬taining power since there are no pivot holes for the detent or other holes for the returning spring. It must have had sun-and-planet maintaining power similar to John Arnold No. 11 (registration no. 1958,1201.1848). This probably failed at some stage and a repairer carried out the conversion in the simplest possible way. The original fusee great wheel has been used for the going barrel. The holes for the clicks, their springs and the internally cut wheel rivets are all visible. The wheel has been turned out on the inside for the barrel, fixed in place with soft solder. On the back plate two sinks have been turned to receive the barrel arbor steel clickwork.
Going train: Arranged to drive the centre seconds hand from a pinion on the escape arbor. The lower pivot of the escape arbor passes through a hole in the front plate and the pivot is supported by jewelling in a small cock on the dial side. Enough of the gilding is intact and together with the jewelling indicates that all this is original. Each wheel has a turned ring on both sides at the roots of the teeth, and all are gilded, though little remains on the great wheel except on the teeth.
Jewelling: The fourth and escapement arbor pivots in pierced jewels. The escape wheel arbor and balance staff lower jewels with endstones in wedged settings, the upper escape jewel with an endstone set blind in a brass plate. The balance cock endstone and setting not original. Where original, the pierced jewels are flat on both sides and without oil sinks. A slight chamfer at each end of the hole is the only reservoir for oil. Both balance staff pierced jewels have been replaced. The locking face of the detent and the impulse and discharge rollers on the balance staff jewelled with dovetailed jewels on their working surfaces. The discharge roller jewel parallel to the staff and standing proud on the lower pivot side of the roller. The impulse jewel set diagonally in the pallet.
Escapement: Similar to John Arnold No. 29 registration no. 1958,1201.1838, but with different safety action. Steel escape wheel polished on both sides, the teeth leaning towards a radial to their tips, the rim section between the teeth polished in a swing tool, the crossings polished by hand. The detent passing spring and locking pallet of polished steel, almost poised, slightly tail heavy, returned by a short spring projecting from the potence and banked by a screw in the side of the potence.
Balance: Later Arnold 'double T' in substitution for the original plain balance and compensation curb, and almost certainly changed by Arnold himself. There are no supporting trammels for the rods carrying the gold compensation weights nor any sign of an amplitude limiting device (see registration no. 1958,1201.18396). Arnold was using the 'double T' balance between about 1778 and 1780. The gold compensation weights are surprisingly far out on the balance. Diam. of rim 255 mm. h. 072 mm.
Balance spring: Previously a spiral spring, now a blued steel helical spring of 6| turns without terminal curves. This is particularly interesting as it illustrates the stage before Arnold invented terminal curves. Both the collet and stud pinnings are at the same radius as the spring which wobbles violently when set in motion. The very crude stud is obviously not original.
Balance cock: The foot cut away to clear the going barrel ratchet and clickwork. Various empty holes around the jewelling were probably used for the screws and pins of the original compensation curb and balance spring stud.
Great wheel (fusee or going barrel) 50 teeth
Centre pinion 12 leaves, wheel 75 teeth, no crossings
Third pinion 8 leaves, wheel 64 teeth, 4 arms
Fourth pinion 8 leaves, wheel 64 teeth, 4 arms
Escape pinion 8 leaves, wheel 15 teeth, 4 arms
Beats per hour: 18,000
Cannon pinion 12 leaves, Minute pinion 14 leaves,
Hour wheel 42 teeth, Minute wheel 48 teeth
A pinion of 10 leaves on the escape arbor drives a wheel of 100 teeth, carrying the centre seconds hand and running loosely on the cannon pinion.
Provenance: By purchase.
Note: This is the earliest known surviving pocket chronometer by John Arnold.
- Not on display
- Exhibition history
1998-1999 04 Dec-18 Apr, London, National Maritime Museum, Arnold & Earnshaw, Pioneers of the Chronometer
Latest: 5 (Jan 2018) small patches of active rust. ####
4 (Oct 1995) Minute hand broken off at boss.
- Acquisition date
- Britain, Europe and Prehistory
- Registration number