- Museum number
POCKET-CHRONOMETER MOVEMENT WITH SPRING DETENT ESCAPEMENT.
Peto cross detent escapement; 3-arm balance with amplitude limiting device.
- Production date
Diameter: 47.30 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. 1800
Signature: 'No. 518 Brockbank's'. On the barrel bridge in characteristic script 'LONDON'.
Dial and hands: Flat enamel dial, cracked, fixed to the dial plate by short feet and pins. The minute and seconds markings were made with a diamond in a dividing engine on rings previously turned in the enamel. Black enamel was later applied over the markings and the dial retired. The circles scribed in the white enamel are faintly visible to the naked eye but the actual divisions under the black enamel are only discernible with a ten times or greater magnification. Roman hour numerals, Arabic minute numerals, subsidiary seconds at 6 o'clock, and signed 'BROCKBANKS LONDON' in small capitals in the upper half. The letter s fired in the back.
Flat red gold minute and hour hands, blued steel seconds hand.
Dial plate diam. 47.5 mm; front plate diam. 44.5 mm; back plate diam. 44.1 mm; frame h. 6.7 mm. (1)
Frame: Full plate with four turned pillars, the back plate secured by blued steel screws. Detachable barrel bridge. The dial plate positioned by three short turned pillars not drilled for pins, but screwed by three radial screws. No dust cover and no provision for one. Otherwise similar to registration no. 1958,1201.1722.
The barrel bridge, back plate and balance cock have all been altered to accommodate the balance and amplitude limiting device. A hole has been drilled in the back plate to allow access to the maintaining power spring foot screw. 72 scratched on the front plate.
Fusee: See registration no. 1958,1201.1722.
Going train: Normal 18,000 beats per hour train. The great wheel has the unusual number of 66 teeth.
Jewelling: The pivots of the third, fourth and escapement arbors run in jewelled bearings, the escapement with endstones. A diamond endstone on the balance cock set in a blued steel ring. The fourth arbor jewel in the back plate a large milky ruby.
Escapement: Peto cross detent escapement, with flat brass escape wheel, not gilded, the teeth raked forwards about 24°. The impulse pallet jewelled with the impulse plane on a radial, the usual jewelled discharge roller. Steel detent, the top of its foot polished, the rest left grey, with an triangular ruby locking stone. Gold passing spring pinned in a square hole in a polished steel arm screwed to the underside of the top plate. A steel block with an adjusting screw to move the passing spring towards or away from the balance staff by bending the steel arm fixed to the balance potence foot (see registration no. 1958,1201.1723).
Balance: Bimetallic three-armed balance with brass segment compensation weights fixed with grained steel plates and polished screws. At the end of each arm is screwed a bracket carrying a screw to limit the outward movement of each rim. Steel timing screws set near the fixed ends of each rim. Broken off screws by the end of each arm also. Rim diam. 17.4 mm, h. 12.4 mm.
Balance spring: Blued steel helical spring of 4½ turns, with terminal curves, the upper rather short, pinned to the shaped steel collet in a square hole. The radially adjustable overhanging steel stud screwed to the back plate, and having a specially shaped block to clamp the upper terminal curve secured by a screw. Brockbanks amplitude limiting device operated by the upper coils of the balance spring. If the amplitude of the balance and spring becomes excessive the coils of the balance spring bear against a block on a pivoted arm. The arm turns against a light return spring until part of the block moves into the path of a steel stud screwed to one of the balance arms, thus preventing the balance from turning further.
Balance cock: Heavily engraved decoration all over the upper surface similar to registration no. 1958,1201.1723.
Great wheel (fusee) 66
Centre pinion 12 leaves, wheel 64 teeth, no crossings
Third pinion 8 leaves, wheel 60 teeth, 4 arms
Fourth pinion 8 leaves, wheel 70 teeth, 4 arms
Escape pinion 7 leaves, wheel 15 teeth, 3 arms
Beats per hour: 18,000
Cannon pinion 16 leaves, minute pinion 20 leaves
Hour wheel 60 teeth, minute wheel 64 teeth
The polished steel minute pinion adjusted on to a brass tube on the minute wheel.
Provenance: Ilbert Collection; purchased from Malcolm Gardner in 1932.
(1) This movement is similar to registration no. 1958,1201.1838.
No. 519 is known and has the hallmark for 1801.
- Not on display
Latest: 3 (2017)
4 (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.1589 (Ilbert Collection)
Previous owner/ex-collection number: L175 (Ilbert Ledger)