A wall mounted astronomical regulator with Airy’s spring detent escapement, manufactured by E. Dent & Co. No. 1906, c.1870

Catalogue description — Royal Museums Greenwich

This clock was used as The Sidereal Standard at the Royal Observatory, Greenwich for over 50 years until it was superseded in 1922 by the Cottingham-Riefler. It has a highly sophisticated detached detent escapement designed by the seventh Astronomer Royal, Sir George Airy. One of the first of its type, the pendulum compensates for temperature and barometric pressure changes. The present glazed hood and door were replacements made for the clock when it was exhibited at the 1951 Festival of Britain. The four-wheel movement is enclosed by substantial rectangular plates, united by stout cylindrical pillars, and is mounted to a substantial cast bronze bracket. The great wheel has seven crossings and is mounted to the barrel arbor in conjunction with Harrison’s maintaining power. The centre and third wheel have six straight crossings and are mounted to high numbered pinions. The escape wheel is of the paddle-wheel type, formed from a single piece of brass with four curved crossings. The escapement is a form of George Biddell Airy’s detent escapement. The crutch is fitted with a fine regulation device (also designed by The Astronomer Royal) in the form of a brass block, suspended on a threaded rod that is moved up or down by turning a hand screw. This device enabled the clock’s timekeeping to be fine tuned without stopping the pendulum or adjusting the length of the suspension. The movement is finely finished with lacquered curling on all flat surfaces. The movement is fitted with two sets of electrical contacts: the first pair is closed by a 30 toothed wheel mounted to the escape wheel arbor, and visible through a circular aperture in the dial. The second set of contacts send out an electrical pulse every 30 seconds; the contacts are closed by two ruby pallets mounted behind the escape pinion. The black painted driving weight is separated from the temperature compensated pendulum by a thin mahogany baffle board; this is to avoid any air disturbance as the weight falls past the pendulum bob. In fact this regulator was so reliable in its rate, it was noted that a rise in barometric pressure equal to one inch of mercury led to a daily loss of 0.3 seconds. In 1873 Airy wrote to Dent proposing the barometric compensation that can be seen on the clock today. Inside the weight chamber is a mercury barometer tube and the float connects to a pivoted ‘see-saw’ like arm carrying a permanent magnet. Barometric changes will change the magnets position relative to the permanent magnet on the pendulum and by doing so will increase or lessen its influence on the clock’s rate. The mahogany case is largely in its original state, though the hood and front door were replaced with fully glazed replacements prior to exhibition at the festival of Britain on 1951. The nominal 12 inch silvered dial is laid out with an outer minute circle, enclosing the seconds and 24 hour subsidiaries which are both situated in the upper half of the dial plate. The lower half is signed by the manufacturer and discretely dated above the 30 minute mark. Due to the action of the escapement, the clock only beats every two seconds. The electrical contacts behind the dial were fitted in 1880 and the signal they produced was used to regulate another clock within the observatory. Dent 1906 was used as The Sidereal Standard at the Royal Observatory, Greenwich for over 50 years until it was superseded in 1922 by the Cottingham-Riefler.