- Museum number
- 1958,1201.1839
- Description
-
Half-quarter dumb-repeating pocket chronometer.
Full-plate; fusee; Harrison's maintaining power.
Arnold spring detent escapement, formerly pivoted detent. Arnold 'double T' balance.
White enamel dial, hours I-XII, subsidiary seconds 15,30,45,60.
Gold hands, seconds hand lacking tail
Gold case, engraved; Hallmark and maker's mark TP; gold chatelaine marked MT
with four seals and four seals.
- Production date
- 1778 (case)
- Dimensions
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Diameter: 45.70 millimetres (back plate)
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Diameter: 62 millimetres (case)
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Diameter: 50.40 millimetres (dial plate)
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Diameter: 46.70 millimetres (front plate)
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Height: 27 millimetres (case)
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Height: 41.50 millimetres (largest seal)
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Height: 30.20 millimetres (smallest seal)
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Length: 180 millimetres (chatelaine)
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Length: 30.90 millimetres (largest seal)
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Length: 23.80 millimetres (smallest seal)
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Thickness: 8.60 millimetres (movement)
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Width: 24.10 millimetres (largest seal)
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Width: 18.80 millimetres (smallest seal)
- $Inscriptions
-
-
- Curator's comments
- Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990)
Made by John Arnold, 1778
Chronometer
Signature: Engraved on the back plate 'John Arnold London Invt et Fecit No 37'.
Case: Gold consular case with pump pendant to operate the half/ quarter dumb repeating work. The edge of the bezel and back with an engraved decoration of leaf and flower design, maker's mark TP (possibly Thomas Primat)(1) and London hallmark for 1778. A push-piece at 8 o'clock and catch opposite. Diam. 62 mm, h. 27 mm.
Dial and hands: Flat enamel dial. The dial attached to the gilded brass dial plate by short feet and pins.
Red gold hands, seconds hand missing its tail.
Movement:
Dial plate diam. 50.4 mm; front plate diam. 46.7 mm; back plate diam. 45.7 mm; frame h. 8.6 mm.
Frame: Full plate with four turned pillars, the back plate retained by pins. Both the going and repeating trains as well as the repeating hammers pivoted between the plates. The remainder of the Stogden-type half/quarter repeating work mounted under the dial. Both plates and pillars made of gilded brass.
Going train: All the wheels are gilded, and the pinions polished to a high standard.
Fusee: Key wound and provided with maintaining power, this with a brass ratchet wheel and blued steel detent pivoted between the plates. The return spring for the maintaining detent is a short piece of steel wire pointing down from the back plate. The setting-up-work for the mainspring is between the barrel and the front plate. The polished steel click and spring are made in one piece. English stop-work.
Jewelling: All the going train pivots including the fusee are run in jewels. These are flat and practically without oil sinks, and set in brass settings held by screws in recessed holes in the plates. The escape wheel and balance jewels have endstones, the escape arbor jewel faces are not curved towards the endstones. The impulse and discharge rollers and the detent are also jewelled.
Escapement: Originally a pivoted detent escapement was fitted and the holes for this were left in place when the watch was modified to Arnold's spring detent. This could well have been done by Arnold, and with great care, because although the slot for the detent is not gilded there is no marking of the gilding round it. The detent banked by a small screw set in the back plate.
Brass escape wheel with the impulse surfaces of the teeth slightly cut where they act on the corner of the jewelled impulse pallet. Steel detent finished grey except for the polished top of the foot. The passing spring also of steel and riveted on.
Balance: The balance is made up on a solid polished steel rim and single cross-bar and known as the 'double T' type from the form of compensation. Two bimetallic strips, with a peg at each end sprung into holes in the rim, are mounted parallel to the crossbar of the balance. A steel rod is fixed at the centre of each bimetallic strip, thus making up the T-shape. Each steel rod passes freely through a hole in the rim and then has a gold weight fixed to its free end. There are steps in the rim to reduce the air resistance. Diam. 260 mm, h. 085 mm.
Changes in temperature cause the bimetallic strips to bow to a greater or lesser extent, and move the weights towards or away from the centre of the balance. The change in the moment of inertia of the balance so produced compensates for the changes in elasticity of the balance spring, and for dimensional changes.
The steel piece crossing over one of the bimetallic strips is an amplitude limiting device operated by the lower terminal curve of the balance spring. This part of the spring passes through a hole in the steel piece which is itself mounted with freedom to move towards or away from the centre of the balance. One end of the steel piece runs through a hole in a short pillar on the balance rim, the other in a slot in a brass peg set in the boss of the balance. If the balance turns clockwise the steel piece is pushed out from the centre of the balance. Should the amplitude then exceed a certain value, the outer end of the steel piece contacts a pin pointing downwards from the overhanging part of the balance cock. Since this device upsets the isochronal adjustment of the balance spring it was often removed from balances to which it was fitted.
John Arnold's famous chronometer No. 36, which was tried at Greenwich in 1779 and 1780 and gave such an outstanding performance, was fitted with a 'double T' balance (see Gould 1923, pp. 110-12).
Balance spring: Blued steel helical spring having just under 5 complete turns with a terminal curve at each end. The upper end pinned to an adjustable gilded brass overhanging stud screwed to the top plate.
Going train counts:
Great wheel (fusee) 60 teeth
Centre pinion 10 leaves, wheel 80 teeth, no crossings
Third pinion 8 leaves, wheel 48 teeth, 4 arms
Fourth pinion 8 leaves, wheel 60 teeth, 4 arms
Escape pinion 6 leaves, wheel 15 teeth, 4 arms
Beats per hour: 18,000
Repeating train counts:
Pinions of 6 leaves.
Wheels 48, 38, 32, 30 and 26 teeth respectively
Motion work:
Cannon pinion 4 leaves, minute wheel (square tooth form)
48 teeth Minute pinion 36 teeth, hour wheel 36 teeth
With this watch, and possibly contemporary, is a gold chatelaine bearing the mark, MT, of an early owner, with four seals, as follows:
(i) A lozenge pattern set in an agate surround.
(ii) A carnelian seal of the arms and crest of the Edmonds family of Lidington, Rutland, Truro, Cornwall, and Winslow, Bucks,
(iii) A lava seal of the Edmonds crest and the letter E.
(iv) A carnelian seal with the Edmonds crest and the monogram RE.
Provenance: Ilbert Collection; purchased by Ilbert c. 1927 from Percy Webster.
Note:
(1) Thomas Primat's mark was registered 1777 and his address given as 3 New Compton St, Soho.
- Location
- Not on display
- Exhibition history
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Exhibited:
2015 18 Feb-14 Jun, Germany, Staatliche Kunstammlungen Dresden, Simple and Perfect: Saxony's Path into the International World of Watch-Making
- Condition
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Latest: 5 (Jan 2018) small amounts of active rust on balance ###. Monitor glass for weeping
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2 (2017)
- Acquisition date
- 1958
- Department
- Britain, Europe and Prehistory
- Registration number
- 1958,1201.1839
- Additional IDs
-
Previous owner/ex-collection number: C24 (Ilbert Ledger)
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Previous owner/ex-collection number: CAI.1839 (Ilbert Collection)
