Silver-cased subsidiary seconds pocket-chronometer with spring detent escapement

Description — British Museum

Pocket chronometer. Full-plate; fusee; Harrison's maintaining power. Arnold spring detent escapement; Arnold 'Z' balance White enamel dial, hours I-XII, subsidiary seconds 10-60, signed. Blued-steel hands. Silver case with hallmark and casemaker's mark.

Curator's comments — British Museum

Comment from Anthony G. Randall and Richard Good, Catalogue of Watches in the British Museum. Vol. VI (1990) Made by John Roger Arnold, 1818 Chronometer Signature: 'Jn° R. Arnold London Invt et Fecit No. 2162. The 'London' engraved in Gothic lettering and the number positioned on the barrel bridge. Case: Silver case with heavy pendant, the front glazed bezel, and back, snapping on to the middle. London hallmark for 1818, the maker's mark T.H. Diam. 55.5 mm, overall h. 24.7 mm. Dial and hands: The flat enamel dial has the signature in gothic letters. The seconds hand a later addition. The minute and hour hands of blued steel, the minute hand having a round hole in its boss and adjusted flush on the end of the cannon pinion. The hands can only be set by pushing the minute hand itself. This is unlike any of the previous examples in which the cannon pinion has an extended square onto which the minute hand is adjusted, leaving room to apply the key for setting to time. Movement: Dial plate diameter 49.0 mm; front plate diam. 45.0 mm; back plate diam. 44.4 mm; frame h. 8.6 mm. Frame: Full plate with detachable barrel bridge with four turned pillars, the back plate retained by pins. The movement hinge screwed to the dial plate at 12 o'clock, the catch opposite attached to the front plate with its return spring. The dial plate apparently mercurial gilded and secured to the front plate by three short feet and pins. The gilding very pale, possibly electro-gilded over the original. Note also the rounded foot of the dust pipe round the winding square. The foot of the balance potence has an unusual moulding along the edge nearest the edge of the back plate; the potence has been moved. The dial with two feet and pins to secure it to the dial plate. The frame screws on the front plate under the dial all polished flat. The maintaining detent and case catch springs left with a rough-grained finish. Fusee: Keywind fusee fitted with stop-work and maintaining power, brass maintaining ratchet wheel. The blued steel maintaining detent pivoted between the plates, with its grained steel spring screwed to the inside of the front plate. The setting-up ratchet wheel squared to the extended barrel arbor on the barrel bridge. The wheel, its click and the return spring polished. Going train: Train similar to registration nos 1958,1201.1842 and 1958,1201.1583, but the escape wheel of 16 teeth gives the unusual number of 19,200 beats per hour. Jewelling: The fourth arbor lower pivot and those of the escapement in pierced jewels, the escapement with endstones. A diamond endstone set in a blued steel plate on the balance cock. Top escape jewel setting not original. Escapement: Arnold spring detent escapement with brass escape wheel of sixteen teeth mounted on a polished steel collet on the escape pinion arbor. The acting surfaces of the escape wheel slightly cut. Normal jewelled discharge roller. The impulse roller jewelled radially (see registration no. 1958,1201.1581). The grey steel detent with square foot, and bevelled sides as in Swiss work. The banking for the detent by a round steel plate held by an eccentric screw to the back plate (see registration no. 1958,1201.1584). All the pivots of the escapement of modern Swiss form. Gold passing spring riveted to detent. Balance: 'z' type with separate bimetallic rims screwed to upstanding blocks on the ends of the crossbar. Timing nuts on studs set in the blocks on the ends of the crossbar. Poising screws, one missing, the other very small, set radially in holes near the free ends of the rims. Diam. 19.5 mm, h. 2.6 mm. Balance spring: Later flat spiral blued steel spring with terminal curve pinned to an overhanging adjustable stud screwed to the back plate at the end of the barrel bridge. This probably replaces an original helical spring. Balance cock: Plain and without decoration, and with a small hole drilled on the centre line about halfway along the overhanging portion. A new adjustable sliding stud fitted to the existing foot. Going-train counts: 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 80 teeth, 4 arms Escape pinion 8 leaves, wheel 16 teeth, 4 arms Beats per hour: 19,200 Motion work: Cannon pinion 12 leaves, minute pinion 16 leaves Hour wheel 48 teeth, minute wheel 48 teeth. Provenance: Ilbert Collection, purchased by Ilbert from Willis Room, The Old Wells Collection. Note: Most of the steel parts show signs of refinishing, suggesting that the watch has at some time been in water and the steel parts rusted as a result. The edges of many of these parts still show extensive signs of rusting. This would also explain the replacement detent, the spiral steel balance spring, and the electro-gilding. The unusual slope of the balance potence foot moulding remains a mystery.