- Museum number
SILVER-CASED SUBSIDIARY SECONDS POCKET-CHRONOMETER WITH SPRING DETENT ESCAPEMENT AND DUST-CAP.
Spring escapement; early Earnshaw type.
Later silver case.
- Production date
Diameter: 55 millimetres (case)
Diameter: 48.80 millimetres (dial)
Height: 80 millimetres (to top of bow)
Thickness: 23.80 millimetres
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by Brockbank, c. 1798
Signature: 'No 919 Brockbank's London '. On the dial below XII, 'BROCKBANKS LONDON'.
Case: Later continental silver case, by the marks, probably Russian, with fixed dome, the glazed bezel and back hinged to the middle, the joint broken. No push-pieces in the pendant or fly springs in the case. Diam. 55 mm, h. over glass 21.5 mm.
Dial and hands: Flat enamel dial similar to registration nos 1958,1201.1722 and 1958,1201.1589. The divided circles for minutes and seconds scribed with a sharp point, the divisions applied in enamel afterwards. The dial secured to the dial plate by three feet and pins.
Flat gold minute and hour hands probably not the original, the seconds hand missing.
Dial plate diam. 49.5 mm; front plate diam. 46.7 mm; back plate diam. 44.0 mm; frame h. 7.3 mm.
Frame: Full plate construction with four turned pillars, the back plate retained by pins, detachable barrel bridge. The dial plate carrying the movement hinge secured to the front plate by three radial screws in the edge. The case catch and spring on the front plate under the dial. The back plate slotted for the detent, the former has broken and been repaired as a result of being weakened by this process. The balance potence very small and with an unusually small foot. The escape wheel pivoted between the back plate and a potence passing into a rectangular hole in the front plate but screwed and steady pinned to the underside of the back plate. All the brass parts gilded.
Fusee: Keywind fusee with the usual stop-work and maintaining power. The maintaining ratchet wheel steel, the detent spring passing over the edge of the great wheel. The barrel arbor squared on the lower end only, the setting-up-work mounted on the front plate under the dial.
Going train: It is unusual for a pocket chronometer to have a 14,400 beats per hour train. This is produced by the escape wheel of twelve teeth. The 60:1 ratio between the centre and fourth arbors is achieved in an unusual way, the fourth pinion having more leaves than the third. The wheels are all nicely gilded, the pinions well polished and finished. The face of the centre pinion is polished in a similar way to those of Josiah Emery.
Jewelling: The pivots of the escape arbor in pierced jewels, those of the balance in pierced jewels with endstones. The balance potence endstone set in a wedge-shaped slip of brass, that in the balance cock a diamond set in a polished steel ring.
Escapement: Earnshaw type of spring detent escapement with flat steel twelve-teeth escape wheel, the radiused backs of the teeth only polished. The wheel mounted on a brass collet. The detent with divided return spring screwed to the end of a brass block, itself screwed and pinned to the underside of the back plate. The short steel passing spring screwed on, its free end resting against the upstanding end of the detent horn. Ruby locking stone with two flat surfaces at right angles, joined by a radius. The detent banked against a steel screw set in a steel plate screwed to the upper side of the back plate. The steel impulse roller with radial jewel, the whole resembling a John Arnold impulse roller. The steel discharge roller slotted to take a projecting rectangular discharge jewel with slightly rounded end. The detent planted with its centre line well inside a tangent to a locked tooth.
Balance: Plain steel three-armed balance polished on both sides, the underside of the rim filed for poising, held with two steel screws on a brass collet on the balance staff. Diam. 21.0 mm, h. 0.58 mm.
Balance spring: Open coiled spiral spring of 65 turns, pinned to an overhanging gilded brass stud, and to a round brass collet. Blued steel index on the balance cock, the pins projecting down through a slot in the overhanging portion of the cock.
Amplitude limiting device: Between the brass collet on the balance staff and the impulse roller, a brass ring is fitted with a projecting peg. A hole is drilled into the side of the clearance hole in the back plate for the balance, presumably for a corresponding peg to limit the balance amplitude to half a turn in either direction. This peg is missing, leaving the empty hole.
Great wheel (fusee) 68 teeth
Centre pinion 12 leaves, wheel 80 teeth, no crossings
Third pinion 8 leaves, wheel 60 teeth, 4 arms
Fourth pinion 10 leaves, wheel 70 teeth, 4 arms
Escape pinion 7 leaves, wheel 12 teeth, 4 arms
Beats per hour: 14,400
Cannon pinion 12 leaves, minute pinion 10 leaves
Hour wheel 40 teeth, minute wheel 36 teeth
Provenance: Ilbert Collection; purchased by Ilbert from Wartski in 1930.
- Not on display
- Exhibition history
2015 18 Feb-14 Jun, Germany, Staatliche Kunstammlungen Dresden, Simple and Perfect: Saxony's Path into the International World of Watch-Making
Latest: 3 (Apr 2019)
5 (Jan 2019) weeping (glass out but in tray)
5 (2017) weeping
2 (Aug 2014) Movement clean and no rust, but issues as below. Also suspected weeping glass. Broken back plate (held together by barrel bridge). Damage to barrel winding work under dial.
2 (Oct 1995) Seconds hand missing. Dial chipped. Case hinge broken. Glass loose.
- 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.1850 (Ilbert Collection)
Previous owner/ex-collection number: F55 (Ilbert Ledger)