- Museum number
POCKET-CHRONOMETER MOVEMENT WITH SPRING DETENT ESCAPEMENT [TEMPERATURE COMPENSATED BALANCE BENEATH GLAZED APERTURE IN DIAL].
Pocket chronometer, movement and dial.
Full-plate; unusual construction with balance under dial; going-barrel.
Arnold spring detent escapement; Earnshaw compensation balance; duo-in-uno balance spring.
Dial missing but dial-plate and glazed ring remain. Formerly with aperture in dial to show balance.
Paper mock-up showing form of dial.
- Production date
Diameter: 42.90 millimetres (back-plate)
Diameter: 50.80 millimetres (dial-plate)
Thickness: 13.70 millimetres
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by John Roger Arnold, c. 1820 (No. 2173 has the hallmark for 1819)
Signature: 'Jn° R. Arnold London Invt. et Fecit No. 2175'.
Dial and hands: The dial missing, but the dial plate still in place fixed to the front plate by three steel screws. The balance was originally visible through a glazed aperture in the dial, the rim and glass still extant.
Blued steel offset minute and hour hands, and sweep centre seconds hand. The sweep seconds hand with steel pipe and polished circular counterpoise end.
Dial plate diam. 51.0mm; front plate diam. 43.8mm; back plate diam. 43.1 mm; frame h. 7.8 mm.
Frame: Unusual full plate construction, three turned pillars supporting the back plate retained by pins. The detent and a cock for the balance lower bearing mounted on the back plate. The balance itself mounted under the dial and supported by a somewhat flexible cock screwed to the dial side of the front plate. The back plate also cut away to clear the going barrel, pivoted and standing off the front plate. A separate cock under the dial for the third and fourth wheel pivots on that side. A hole through the dial plate, with masking bridge to prevent ingress of dust, for ease of timing screw adjustment.
Going barrel: Mounted on the front plate with its ratchet wheel under a bridge on the dial side.(1) The barrel arbor squared on both ends, one end for winding and the other on the front plate and passing into a squared hole in the ratchet wheel. The ratchet wheel itself having a short pipe providing a pivot in the barrel bridge. The squared barrel arbor and the ratchet wheel pipe are pinned tightly together. A four-turn Geneva stop-work fitted on the barrel cover. Dust pipe with its foot fixed to the barrel cover.
Going train: So arranged that the fourth wheel is at the centre and can carry the sweep centre seconds hand. The centre wheel is offset but carries the canon pinion and drives the motion work in the ordinary way. An unusually fast train of 21,600 causing the seconds hand to advance three times per second.
Jewelling: The fourth pivot on the dial side running in a jewelled hole. The pivots of the escapement running in jewelled holes with endstones. Both balance endstones are diamond, the one at the dial side loose, not in a setting. The detent, impulse and discharge rollers jewelled at their working surfaces.
Escapement: Arnold spring detent escapement, the brass escape wheel having the usual number of eighteen teeth. The impulse surfaces of the escape wheel teeth Only slightly marked, suggesting that the watch has had little use. An eccentric plate and screw are banking for the detent.
Balance: Earnshaw-type balance, the steel part of the rims and the crossbar are part of the same piece of steel. Adjustable brass segment compensation weights held by round-headed steel screws. Additional adjustable weights made of screws fixed to the crossbar. Timing screws set radially at an angle in the shorter sections of the bimetallic rim, which are bent inwards. Diam. 17.0 mm, h. 1.23 mm.
Balance spring: Duo-in-uno blued steel spring pinned to the adjustable steel overhanging stud with a square section pin, and to the steel collet with a D-shaped pin.
Great wheel (barrel) 64 teeth
Offset centre pinion 8 leaves, and wheel 64 teeth, no crossings
Third pinion 8 leaves, wheel 60 teeth, 4 arms
Fourth pinion 8 leaves, wheel 80 teeth, 4 arms
Escape pinion 8 leaves, wheel 18 teeth, 4 arms
Beats per hour: 21,600
Cannon pinion 10 leaves, minute pinion 12 leaves
Hour wheel 36 teeth, minute wheel 40 teeth.
Provenance: Ilbert Collection, purchased by Ilbert from Jauncey in 1936.
(1) J. R. Arnold began to use the going barrel in his marine chronometers in 1807.
- Not on display
Latest: 3 (2017)
3 (Oct 1995)
- Acquisition date
- Acquisition notes
- Following the successful acquisition of the celebrated Ilbert collection of clocks (1958,1006 collection), prints and other related materials made possible by the generous donation of funds by Gilbert Edgar CBE Ilbert's watches were then acquired using funds provided by Gilbert Edgar, public donations and government funds.
- Britain, Europe and Prehistory
- Registration number
- Additional IDs
Previous owner/ex-collection number: CAI.1584 (Ilbert Collection)
Previous owner/ex-collection number: P101 (Ilbert Ledger)