Thomas Mercer Survey Chronometer with Audible Signal

A Mercer's survey chronometer, two-day duration with Arnold…
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A Mercer's survey chronometer, two-day duration with Arnold type spring detent escapement and fusee and chain movement, silvered and engraved dial marked 'Thomas Mercer Maker to the Admiralty, The Indian Colonial & USA Governments, London & St. Albans, 12781', blued steel hands. Hours are numbered 0-23 with Arabic numerals. Bayonet bezel with shuttered winding hole in the glass. Dial is cut away to show a 59 tooth contact lifting wheel mounted on the seconds arbor and a contact blade which is lifted each second except for the 59th in each minute to enable an audible signal in earphones each second with the silence being a warning of the approach of the full minute. Mounted in blackened brass cylindrical case with external battery and microphone contacts and stop/start lever and encased in heavy leather carrying case with strap, and pouch for safety ratchet winding key. Originally purchased by New Zealand Government in 1925 (c.f. Mercer: Mercer Chronometers p.210.)

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  • Bezel - On a clock or watch, the bezel is the metal frame into which the watch or clock glass is fitted. In clocks, the bezel may include a hinge and a flange, in effect a door to the face of the clock. In jewellery the bezel is a band of metal with a projecting lip that holds the gemstone in its setting.
  • Movement - The technical name for the workings of a clock or watch, and does not include the dial or case.
  • Fusee - The fusee movement was used in clocks and pocket watches from the mid 17th century. The fusee is a cone shaped drum within the works that is linked to the barrel of the spring, usually by a length of chain.

    As the mainspring loses its tension over time, the cone shaped barrel compensates for this by increasing the tension, by pulling the mainspring tighter, thus ensuring the time remains constant.

    Use of the fusee in clocks was superseded by the "going barrel" in the mid 19th century and for pocket watches at the beginning of the 19th century.

    The fusee continued to be used in marine chronometers until the 1970s.

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