- Museum number
THREE-QUARTER PLATE POCKET-CHRONOMETER MOVEMENT WITH PIVOTED DETENT ESCAPEMENT.
Pivoted detent escapement; up and down; Powell's going barrel (?); keyless.
Diameter: 43.80 millimetres
Thickness: 12.30 millimetres
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by John Forrest, c. 1870
Signature: On the back plate 'John Forrest London no 5478 Chronometer maker to the Admiralty'. On the dial 'CHRONOMETER 5478'.
Dial and hands: Flat enamel dial with sunk subsidiary seconds dial and sunk centre to the up-and-down dial. The dial attached to the front plate by three feet and pins, one foot machined so as to clear the keyless work spring.
Blued steel hands, the up-and-down hand missing.
Ebauche marks 16, o, 82 (or S2) and underneath 5478.
Front plate diam. 44.5 mm; back plate diam. 42.6 mm; frame th. 9.1 mm.
Frame: Massive half plate construction with three turned pillars, the unusually thick back plate (2.1 mm) secured by recessed blued steel screws. The movement was secured in its case by a peg in the edge of the front plate and two dog screws on the edge of the back plate. All the brass parts gilded.
Mainspring barrel and barrel arbor assembly: A very unusual arrangement: the mainspring is contained in a barrel forming part of a bridge and secured open side down on the front plate. The barrel arbor is pivoted in the front plate and in the barrel. Stop-work, now missing, was fitted on the top of the barrel.
The barrel arbor has an integral winding ratchet wheel at about the middle. The boss to which the spring is hooked is held with a transverse pin to the arbor. Between this boss and the winding ratchet wheel is sandwiched a thin steel wheel with ratchet-shaped teeth on its edge and carrying the clicks and springs to work with the winding ratchet wheel. This wheel is similar in appearance to a fusee maintaining power ratchet wheel and performs the same function; it is free to turn on the arbor. The great wheel fitted with the maintaining power spring is held on the arbor by a collar pinned through the arbor. The whole barrel arbor assembly can be considered as a fusee fitted with maintaining power in which the fusee itself has been replaced by a boss hooked directly to the mainspring. A maintaining detent and spring are provided to work with the maintaining ratchet wheel. This complicated arrangement is to protect the going train in the event of mainspring breakage.
Keyless work: The end of the barrel arbor passing through the front plate is squared and carries a steel winding wheel held on with a pin. This wheel is in permanent mesh with another smaller steel wheel pivoted on a shoulder screw. Both these wheels turn during winding and going. A type of rocking bar keyless work is fitted in which the two smaller rocking bar wheels are held out of mesh with all the other wheels except the central larger wheel. Two springs hold the rocking bar in its middle position. Two push-pieces must have been provided in the original case, one on each side of the pendant, each acting on one end of the rocking bar. When looking at the movement from the front with the dial removed the left-hand push-piece would have caused the rocking bar to tilt clockwise and mesh the wheels for hand setting. There are two intermediate hand-set wheels to convey motion to the minute wheel.
Up-and-down indicator: On the end of the barrel arbor is fitted a small pinion meshing with a brass wheel pivoting on a pillar nearby. An indicator hand was fitted on a cannon integral with the brass wheel. Since the barrel arbor turns in one direction during winding and the other during running this simple system can be used.
Going train: Constructed in the usual way and laid out in a similar way to that for a reversed fusee. The centre arbor hollow for the square-ended arbor carrying the cannon pinion. The brass wheels gilded, the steel pinions and arbors well finished and polished.
Jewelling: The pivots of the third, fourth and escapement arbors in pierced jewels, those of the escape pinion and balance staff with endstones.
Escapement: Pivoted detent escapement, the brass escape wheel not sunk out for lightness, but finished with the upper and lower surfaces burnished. The polished steel impulse roller with the impulse surface slightly rounded and inclined so that the outer end leans towards a radial to the tip. The jewelled steel discharge roller with an adjusting flat, fitted between the impulse roller and the brass collet on which the balance is mounted. The detent pivoted near the edge of the frame and having both the gold passing spring and the leaf return spring pinned into holes in a central boss. The locking stone made from a half-round piece of ruby. Banking provided by a screw set in a polished steel arm overhanging the escape wheel. The end of the screw banks against the locking stone pipe. Locking of the escape wheel is carried out on the tooth after that which has just given impulse, the detent being returned after unlocking by the leaf spring bearing against a brass peg set in a sink in the front place. The pivoting point of the detent is set inside a tangent to the locked tooth so that the detent and horn are straight.
Balance: Bimetallic two-armed balance with a mixture of large gold screws and small steel screws between them. Diam. of rim 14.4 mm, h. 1.68 mm.
Balance spring: Blued steel helical spring of 8½ turns, with terminal curves, free sprung to a polished steel diamond-shaped stud on the balance cock and a brass counterpoised collet.
Great wheel 90 teeth
Centre pinion 12 leaves, wheel 80 teeth, 6 arms
Third pinion 10 leaves, wheel 75 teeth, 6 arms
Fourth pinion 10 leaves, wheel 80 teeth, 6 arms
Escape pinion 8 leaves, wheel 15 teeth, 6 arms
Beats per hour: 18,000
Cannon pinion 10 leaves, minute pinion 10 leaves
Hour wheel 40 teeth, minute wheel 30 teeth
Central rocking bar wheel 56 teeth
Winding rocking bar wheel 28 teeth
Hand set rocking bar wheel 24 teeth
Intermediate winding wheel 18 teeth
Barrel arbor bar wheel 38 teeth
Intermediate hand-set wheels, each 18 teeth
Driving pinion 10 leaves
Wheel carrying the hand 60 teeth
Provenance: Ilbert Collection; purchased by Ilbert from J. W. Benson in 1935.
- Not on display
Latest: 4 (2017) rust (inactive) on hand
4 (Oct 1995) Up-and-down hand missing
- 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.1597 (Ilbert Collection)
Previous owner/ex-collection number: N282 (Ilbert Ledger)