Thomas Mercer, St. Albans c.1965 No.1149
Chronometer master clock ZAA0747

The movement is now devoid of any casing having been acquired as a stand alone movement, but one associated slave dial was acquired with it.

Dial & Hands and Dial Movement The dial and its secondary movement, are entirely separate from the movement of the master clock, but came with the master clock on acquisition, and the two do appear to belong together. The dial itself is of white painted brass and has black painted arabic numerals and five minute-figures, the minutes at the top of the dial marked with a 0. Below the dial centre is marked: MS 122/312, and MERCER / ST ALBANS. ENGLAND. The back of the dial is stamped: 2. There are black-painted spade and poker hour and minute hands, and a red-painted centre-seconds hand with a counterpoised tail.

The dial is fixed to the front of a chromium-plated brass, four pillar, seconds-impulsed movement, designed to incorporate the motion work. The movement originally also incorporated an additional contacting mechanism which is now missing. Centred in the movement is the seconds-impulsed pawl wheel, having 60 triangular teeth and advanced every second by the typical advancing arm with pivoting pawl at the top, actuated by a single electro-magnetic coil mounted low down in the movement. The arbor of this wheel (which has the centre seconds hand mounted on its front pivot) also has a small pinion mounted at the front just inside the front plate. This meshes with a motion-work driving wheel, which has a short arbor running through the front plate and with a steel pinion on the other side, which in turn meshes with the minute wheel. The minute wheel, the tubular centre pipe of which runs on the extended centre seconds pivot, has the minute hand attached to the front of its pipe. A pinion mounted on the front surface of the minute wheel meshes with an intermediate wheel, also having a pinion on its front which then meshes, in conventional motion work layout, with the hour wheel, the pipe of which runs on the minute wheel pipe.

It is uncertain precisely what parts are missing from this movement, but a brass pinion mounted at the rear end of the arbor of the centre seconds pawl wheel evidently pitched with a wheel mounted inside the backplate, at a ratio of about 6:1. Also mounted on the arbor of the centre seconds pawl wheel is a steel snail-cam apparently designed to operate a pair of contacts once a minute. There would appear to have been further parts to the movement mounted at the top and on the back of the existing frame.

Chronometer Movement Vertical, going barrel movement in frosted chromium-plated brass, with three straight cylindrical pillars with inset screws at both ends, the fourth corner having a pillar in the form of a block, supporting a pair of electromagnetic coils (12v, dc). These occupy the lower part of the movement, between the plates, and operate a lever with a pawl, for advancing the auto-winding mechanism on the front plate of the movement. It is the half-minute electrical impulses sent from the clock movement itself which, as well as their other duties, are intended to keep its own mainspring wound, using the half-minute impulsing mechanism, described shortly.

The movement as a whole is nicely finished though not to Mercer’s standard for a marine chronometer at this period. On the back of the movement is screwed a similarly plated horizontal mounting bar, projecting out on both left and right, for fixing the movement to the back of the cabinet into which it would be installed when in use. Both front and back plates, and the mounting bar are machine-engraved near the top edge: 1149, and the front fourth wheel cock is engraved under the foot: 49.

The power source is a going barrel mounted between the plates. The barrel and the barrel cap are scratched inside: 17. The arbor of this barrel projects through the front plate of the movement and has mounted on its front end a winding wheel. The winding wheel has mounted on its periphery a backward-projecting adjustable ‘auto-stop’ peg. The winding wheel is driven by a pinion, mounted on a pawl-wheel which is moved forward one tooth by the advancing pawl of the electromagnetic coils once every half minute.

The part of the barrel arbor in front of the barrel, but still between the plates, is cut with a screw-thread and, threaded on the arbor between the barrel and the front plate, is a stout brass ‘auto-stop’ disc. The front surface of the barrel has a brass post fixed in it, projecting forwards and running freely through a hole in the brass disc, thus carrying the disc with it when the barrel revolves, and preventing the disc from revolving in relation to it. However, should the barrel arbor turn in relation to the barrel, the disc, threaded on the arbor, will move up or down the thread. Under normal running conditions, the train and barrel will revolve at the same rate as the arbor is being wound, so no relative movement would occur. If however the auto-winding mechanism stops advancing the barrel arbor (if the electric supply to the coils fails for example), the chronometer will continue to run for about 4 hours, until the brass disc has wound back down to the end of the threaded arbor, up against the barrel, and the chronometer will stop. If, on the other hand, the chronometer stops but for some reason the half-minute impulsed, auto-winding mechanism continues to wind the arbor (if, for example, it receives its impulses from a secondary source), the brass ‘auto-stop’ disc will wind along the arbor in the other direction, towards the front of the movement.

Between the winding wheel and the front plate, screwed flat on the surface of the front plate, is a beryllium copper sprung blade which has a horizontal extension at the top, reaching across to the left of the front plate, and having an angled end, right next to a pair of normally-closed (conducting) electrical contacts mounted on the side of the plates. At the upper end of the beryllium-copper blade, just before it extends to the left, there is, riveted to it, a steel pin, projecting through a hole in the front plate, and pointing towards the stout brass ‘auto-stop’ disc on the barrel arbor. If the chronometer stops, but the winding continues, the auto-stop disc advances forward and comes into contact with the domed and polished end of the pin. As the disc advances further, it pushes the pin and blade forward. The angled end of the blade extension moves forward with the rest of the upper part of the blade, but does not cause any movement in the electrical contacts as it is merely moving forward, alongside them. However, as the winding continues, and the winding wheel comes clockwise round, the backward-projecting ‘auto-stop’ peg on the wheel, the end of which usually just misses the extension on the blade, now comes up under the extension, which has moved into its path, and pushed it upwards. The angled part on the end of the extension now meets a similarly angled (and insulated) projection on the tip of one of the electrical contacts, and as the winding advances the electrical contacts are opened, and hence the supply to the winding coils is stopped.

In all, the train has five wheels, including the going barrel, the fourth wheel which is a contrate driving the escape wheel on a horizontal escapement platform) and the fifth being the escape wheel. The second wheel and its collet are marked: 49. Mounted on the front of the contrate wheel (which turns once in a minute, but revolves anti-clockwise when viewed from the front), is an electrical contacts wheel, closing a pair of contacts every second, for driving a seconds impulse dial. In addition, on the front of the contrate wheel arbor is mounted an insulated, Paxolin double-snail cam, which acts on a second pair of electrical contacts to provide half-minute pulses.

Escapement, Balance & Spring and Jewelling The Earnshaw-type spring detent escapement is mounted on a horizontal, chromium-plated brass platform across the top edges of the movement plates. The platform is engraved on the top surface: MERCER / ST ALBANS ENGLAND / 1138 , MADE IN ENGLAND , and ELEVEN JEWELS. The balance cock, which has had its chromium plating grained off the top and sides, is stamped underneath 1138. The escapement has Mercer’s steel flat spring detent mounted on the side of a chrome-plated brass block on the platform. The detent has a screwed on, gold passing spring with a clear jewelled locking stone. The impulse roller has a radially set, red impulse jewel and the discharge roller has a similar stone inset. Underneath the balance, pivoted on the upper surface of the platform, is a swivelling brass arm, with a curved beryllium copper blade spring on the end which, when swung round, gently slides under the rim of the balance and brings the balance to a safe halt. An extension to the arm, projecting off the front of the platform, provides the lever by which the stop mechanism is operated, with the finger.

The two-arm plain compensation balance, which is scratched on the underside of one of the arms: 1136, has cylindrical, black-patinated brass compensation weights screw-clamped to the rim segments about 100° from the arms. Brass mean-time nuts are mounted on fixed threads at the end of the arms.

The Palladium helical balance spring has terminals on both ends, the upper terminal with a brass pointer stud screwed to the balance cock. The jewelling, which is mounted in brass settings, extends to the balance and escape wheel pivots, with endstones, the front and rear contrate wheel pivot holes, and the escapement parts as mentioned.

History The master clock movement and dial, with the copies of drawings, were purchased from Mr David Sadler in 1992.