This clock was one of nine 'secondary' clocks commissioned by Astronomer Royal George Airy for use with altazimuth and equatorial instruments for observing the 1874 Transit of Venus. These wall-sized regulators, each costing £27, originally had wood rod pendulums and according to Dent's estimate with specification, steel pallets. When delivered to the Royal Observatory, the series of clocks were put on test on 'the great stand' on the ground floor of the Great Equatorial building, but 'sympathetic influence' caused the tests to be discontinued. This clock was one of three found to be under-compensated for temperature change and was sent back to Dent for modification. Number '2010' was used at one of the British expedition's temporary observatories on Kerguelen Island in the southern Indian Ocean for observing the 1874 transit of Venus, and again by Captain G. Mackinley to observe the 1882 Transit of Venus; this time at the British station at Up Park Camp, Jamaica.

In 1885 it was adapted by E Dent & Co. for use at Devonport dockyard, as part of preliminary trials to establish hourly time signals at primary signal stations at points along the south coast of England. To provide daily time signals to shipping, primarily for rating marine chronometers, it became desirable to provide at primary points throughout the world, daily or hourly means of indicating the exact time. This usually took the form of a ball or similar visual signal mounted at a prominent point within a dockyard or on a headland that was hoisted a few minutes prior to a pre-determined time and dropped exactly at a pre-determined hour of often one o'clock.

Traditionally, the operation of these signals relied on a nearby observatory finding time through the observation of the transit of stars, a relatively complicated, costly, and time-consuming process. With the advent of the galvanic telegraph, it became possible to transmit time signals to remote points from one central point almost instantaneously. In this case, the central point was the mean solar standard clock at the Royal Observatory Greenwich. The Greenwich signal could be used to directly activate remote signals or devices such as the timeball at Deal, in Kent, England, but problems with reliability of the telegraph line prevented this direct system from being widely adopted.

A more reliable, but slightly less precise system was devised that used the Greenwich signal to correct a remote precision clock (regulator), that in turn would activate the nearby signal. This way, if the Greenwich signal failed, the remote clock would continue running, without correction until the Greenwich signal could be re-established. Converting the clock for its new use involved making the apertures in the dial, and fitting the electrical contacts and electro-magnetic correcting device as described above. In addition, the original wood-rod pendulum was replaced with the present zinc and steel type and an auxiliary pendulum, suspended directly behind the primary zinc and steel pendulum. This auxiliary pendulum (now removed) will almost certainly have been of wood; possibly the original was used. The clock case was extended in depth to accommodate this pendulum and the original wooden panel in the backboard. The backboard was removed and replaced with glass to form a partition between the front and new back compartments.

The wooden panel in the trunk door was also replaced with glass. To one side of the rear portion of the case, a solenoid was fitted that operated a latch. Under normal circumstances the latch held the auxiliary pendulum to one side. An external lever just below the trunk door at the centre of the case with an internal arm, enabled the auxiliary pendulum to be manually returned to the latch. Terminals for connecting the various external wires were fitted to the case and a telegraph key placed near the clock to enable the timeball to be dropped at will. A rotary switch near the clock enabled an attendant to operate the magnetic corrector.

At a pre-determined time (normally ten o'clock), contacts on the Greenwich Mean Solar Standard (master) clock would automatically close, sending an electrical impulse to the remote clock (in this case at Devonport dockyard). This would activate the solenoid within rear portion of the remote clock case, releasing the auxiliary pendulum. On being released, the auxiliary pendulum was allowed to swing freely in coincidence with the pendulum on the master clock. Although the auxiliary pendulum was not impulsed in any way, it would continue to swing without its arc diminishing appreciably for many minutes.

An attendant standing by the remote clock at the time of release would note whether the clock was fast or slow of the Greenwich signal. The difference in phase between the auxiliary and primary (normal) pendulums at the time of release would indicate error. The attendant would then bring the primary pendulum into synchronisation with the auxiliary pendulum by means of the electro-magnetic corrector. When both pendulums swung in exact synchronisation the clock had been brought to time and the current to the electro-magnets would be switched off. Later the same day (normally one hour and 20 seconds after the initial release) the third set of contacts on the remote clock (described above) were automatically closed sending a return signal to Greenwich to confirm the clock had been brought to time.

This system continued in use and a record was kept of the error of the daily return signal until 1937. Since then the clock has been partially restored to its original form. The auxiliary pendulum, solenoid, latch and rear portion of the case have been removed and no longer exist. The glass panel in the original backboard has been again replaced with wood and the backboard re-enforced with strips of wood down both sides. The inside of the clock was refurbished and fitted with an oak base by the MoD chronometer workshops at Herstmonceux Castle. The clock was then transferred to the Hydrographic Office at Taunton where it was used as an office furnishing until 13th June 2002, when it was transferred to the Royal Observatory Greenwich.
The main body of the case and the hood are varnished teak. The trunk and hood have hinged, glazed, lockable doors. A brass sight ring is screwed to a mask inside the hood door.

The case, originally intended to be screwed to a balk of wood driven into the ground, is now screwed to an oak base and is floor standing. The front panel in the base is hinged and lockable.

The 288 mm diameter brass dial is painted off-white, it is laid out in a regulator style with concentric Arabic five-minute numerals outside a minute-track, and observatory marks at five-minute intervals. The subsidiary seconds dial above the centre has Arabic ten-second numerals inside a seconds track with observatory marks at five-second intervals. The subsidiary hour dial below the centre has Arabic zero to 23-hour numerals within a circle. The winding aperture is at the 42-minute position, 63 mm from the dial centre. There are three apertures in the dial for observing the adjustment of electrical contacts on the movement.

The dial is signed across the centre 'E. DENT & Co.' and '6, STRAND', 'LONDON', '2010'. Its reverse is painted matt-black, and stamped '2010'. It is secured to the movement via three riveted dial feet and one countersunk steel screw, fixing through the dial into a stud screwed to the movement front plate. The original fourth dial foot has been cut off and filed to a stub.

All three hands are of blued steel riveted to brass collets. The minute hand is internally counterpoised and is retained by a steel collet and taper pin. The seconds and hour hands both have integral counterpoises, and are a friction fit on their respective arbors.

The frame consists of a pair of 207 x 153 mm shouldered brass plates, separated by four slightly tapering brass pillars riveted into the front plate. The back plate is retained by taper-pins. The plates have a spotted finish with curled edges. The entire frame has a green/brown-coloured lacquer finish. The frame and many of the movement components are scratched, or stamped, '151' and/or '10' or '2010'. The movement is screwed through the lower pillars to a one-piece, cast iron movement and pendulum support, screwed to the backboard of the case.

The four-wheel, eight-day duration train has Harrison's maintaining power acting on the great wheel, and stop-work in the form of a stop-piece screwed to the front flange of the barrel. The train engages with a sprung stop-iron screwed to the inside of the front plate when activated by the weight line.

The centre wheel is riveted to its pinion, its arbor has an extended front pivot that carries the minute hand and drives the motion work. The third wheel is riveted to a collet. The escape wheel is riveted to its pinion head and is planted just inside the front plate. It has a short arbor with an extended front pivot, which carries the seconds hand. The seconds arbor pivots between two cocks; one screwed to the inside, and one to the outside of the front plate. The jewelled dead-beat pallets are steel, and screwed to a collet on their arbor, the front pivot of which bears in a block screwed inside the front plate. The brass crutch has an adjustable fork for beat setting.

The great wheel and pulley are not crossed out. Centre and third wheels have five crossings and the escape wheel has four.

A friction tight cannon wheel on the extended centre arbor drives an intermediate wheel and pinion carried on a stud screwed into the front plate, which in turn drives the wheel carrying the hour hand, also carried on a stud on the front plate. Notched disks screwed to the canon wheel and hour wheel control the action of an electrical contact (described below).

The clock is driven by a matt-black painted, cylindrical, brass cased lead weight (approximately 2.5 kg), suspended from a matt-black painted brass pulley and stirrup. The grooved barrel has a keyhole aperture to facilitate replacement of the weight line. An ‘L’ shaped brass bracket screwed to the front plate of the movement provides the second attachment point for the line.

The concentric tubular zinc and steel temperature compensated pendulum has a cylindrical, matt-black painted brass cased bob with a graduated, silvered rating nut and a silvered locking nut.

A steel pointer screwed to the bottom of the rod indicates amplitude on a silvered scale screwed to the backboard of the case.

The movement is fitted with three sets of electrical contacts. Two are mounted on a dovetail slide screwed to the front, upper right hand side of the front plate. These contacts can be set to close once every minute, and once every hour respectively, by small steel contact arms carrying cylindrical jewels mounted on the seconds and minute arbors. Simultaneous closure of both contacts make a circuit on the hour, every hour (this was used to provide a current to drop a timeball within the dockyard at Plymouth).

The third pair of contacts is mounted on a substantial brass arm pivoted between the movement plates. This arm shares a common arbor with an arm mounted outside the front plate which carries a steel 'V' and a brass roller. The action of this arm is controlled by the notched discs screwed to the cannon wheel and hour wheel. Once every 24 hours, notches in the two discs coincide with the steel 'V' and brass roller, allowing the arm with its contact attached to drop slightly, and for the contact to be closed by a second jewelled contact arm mounted on the seconds arbor. This arm is set at a position 20 seconds (120°) behind the first jewelled contact arm, making a circuit that sent an impulse to Greenwich, showing the error of the clock.

Screwed to the backboard of the clock, approximately in line with the centre of the pendulum, is a transverse brass bar, supporting a pair of electro-magnetic coils that embrace the pendulum but allow it to swing freely. These coils act on a steel ‘U’ shaped magnet screwed to the pendulum rod. Introducing a current through the coils produces either an attractive or repulsive force, depending on the direction of the current, thus altering the natural period of the pendulum, allowing the clock to be brought to time.

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