- Museum number
POCKET-CHRONOMETER MOVEMENT WITH SPRING DETENT ESCAPEMENT.
Spring detent escapement.
- Production date
- 1802 - 1807
Diameter: 45.60 millimetres (back-plate)
Diameter: 50.80 millimetres (dial)
Thickness: 19.80 millimetres
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by Thomas Earnshaw, c. 1800
Signature: On the back plate 'Thos Earnshaw Invt et Fecit No 457'. On the barrel bridge 'No° 2760'. On the dial, centrally below the xn 'THOS EARNSHAW INVT ET FECIT 457'.
Dial and hands : Enamel dial with slightly curved edge.
Gold minute and hour hands, both damaged, the steel seconds hand with a pierced disc at the extremity of the long tail.
Dial plate diam. 51.2 mm; front plate diam. 46.6 mm; back plate diam. 45.7 mm; frame h. 8.5 mm.
Frame: Full plate with four turned pillars, the back plate retained by pins. Detachable barrel bridge with turned relief around the hole for the barrel arbor pivot. The dial plate carrying the hinge attached to the front plate by three short pillars and pins. The case catch and return spring mounted on the front plate under the dial. The balance potence foot without the usual moulding. The dial plate only gilded, otherwise the frame left devoid of any gilding. The steel parts left whitish-grey in appearance.
Fusee: Key wind fusee with a dust cup fitted on the winding square. The usual stop-work and maintaining power, the maintaining ratchet wheel brass, the detent return spring a short piece of brass wire pointing down from a hole in the back plate.
The setting-up-work under the dial, the barrel arbor with an extended square at both ends.
Going train: The brass wheels ungilded, the steel pinions with polished leaves, but the ends of the pinions and the arbors left unpolished.
Jewelling: The pivots of the escape pinion arbor and the balance staff in pierced jewels with endstones. The endstone jewels flat on both sides. Diamond endstone in the balance cock set in a blued steel ring. All the other pivots in holes in the brass frame.
Escapement: Typical Earnshaw detent escapement with brass escape wheel, sunk out on the upper side and with characteristic tooth form, the teeth showing signs of wear on the leading edges. The impulse roller left grey except for the polished impulse face, which also shows signs of wear. The usual jewelled discharge roller. The detent appears to be original, the end of the spring clamped against a brass block by a steel screw. This block is riveted to a brass arm and the latter screwed and steady pinned to the underside of the back plate, but overlapping slightly the edge of the plate. A steel screw set in the edge of the back plate can be used to move the detent towards or away from the balance centre by causing the brass arm to bend. The gold passing spring is pinned in a hole in a block integral with the detent body. Banking for the detent is provided by the head of a brass screw set in a separate brass banking piece also screwed and steady pinned to the underside of the back plate. The escape wheel teeth are locked on a polished flat on a steel pin set in the detent. The pin is set so that there is draw between the flat and the wheel teeth, tending to hold the detent in the locked position. It is unusual for a detent such as this to survive because in order to remove and replace it in the watch it is best not to disturb the detent where it is clamped since this will disturb the initial adjustment, but rather to remove the whole detent and block assembly. This is so delicate and difficult to do that the detent is frequently broken, whereupon the whole assembly is discarded and replaced by a new detent with solid foot fixed directly to the underside of the back plate.
Balance: Bimetallic two-armed balance with brass segment weights fixed by screws from the inside. The timing screws at the ends of the arms are probably original but the other four screws set in the bimetallic rims probably later additions when the watch was re-sprung. The steel parts of the bimetallic rims blued. Diam. of rim 19.2 mm, h. 1.73 mm.
Balance spring: Blued steel helical spring of 6½ turns with terminal curves, free sprung to a gold stud on the balance cock and a brass collet on the balance staff. At the time that this watch was made Earnshaw was not using hardened and tempered steel wire for his balance springs. Such springs caused a gradual slowing down of the rate, so it is not unusual to find them replaced by a hardened and tempered spring. In this instance the addition of screws to the balance would seem to indicate that the spring has been replaced although the extra screws could be required because the spring was shortened for some reason.
Great wheel (fusee) 60 teeth
Centre pinion 12 leaves, wheel 75 teeth, no crossings
Third pinion 10 leaves, wheel 64 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 12 leaves, minute pinion 14 leaves
Hour wheel 42 teeth, minute wheel 48 teeth
Provenance: Ilbert Collection, purchased by Ilbert from Jauncey in 1934.
- Not on display
Latest: 3 (2017)
4 (Oct 1995) Hour hand broken.
- 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.1592 (Ilbert Collection)
Previous owner/ex-collection number: N274 (Ilbert Ledger)