THOMAS MERCER No. 19269: A two-day survey chronometer, with 24-hour dial and half-second make/break' electrical contacts, and 'start/stop' mechanism signed and numbered Thomas Mercer 19269, Arabic hour chapters, blued steel hands, subsidiary up-and-down and seconds dials, the latter with cut-away sector for access to the electrical contacts assembly, chrome satin finish to plates and balance cock, Earnshaw escapement, Mercer flatstrip detent to side of banking block, cut bi-metallic balance, circular heat compensation weights, palladium helical balance spring, winding through the dial, 'reverse' tipsy winding key and separate key for 'start/stop' control, the whole within an aluminium drum case with cap, contained in lidded black ebonite sorbo-rubber lined outer carrying box having polished aluminium quarter-round edging. According to the Thomas Mercer site, this clock was developed in 1926 for British Military Survey Work. From the internet I found a statement "Mercer confirms that the 19665 was manufactured in 1951," which would make this one earlier manufacture. Another account mentioned that the start stop switch was added to these clocks circa 1962, which places this example as newer. 

Quoted from the Royal Museum Greenwhich website: "Survey Chronometer. An adapted form of the standard two day chronometer was the survey chronometer. Fitted with electrical contacts and rated to sidereal time, the survey chronometer was used in conjunction with a tape chronograph and, by the date of the NMM’s examples, a 45° Admiralty prismatic Astrolabe – an optical instrument for measuring angular distances between a given star and the zenith (the zenith distance). The very accurate timing of the zenith distance of specific stars enables the precise location of the observer to be found and is used in land survey work, enabling precise latitudes and longitudes to be established. A considerable number were issued by the Admiralty for use in accurate surveying of harbours and coastlines during the twentieth century. In order to be sufficiently precise in the determination of the time of observation, the chronometer is used in conjunction with the tape chronograph. The principal of this instrument is to enable the moment of an astronomical observation to be recorded permanently, in a visual form, enabling an accurate determination, at leisure, after the observation is made."

I have used this clock for display purposes only. Prior to listing, I checked the movement and it ran strong. Because it has not been serviced recently, I did not run it for long. The leather strap (using original buckle) and the lock strike were replaced. I will provide the original parts. Also included in the sale are copies of literature describing the clock's function.

The clock and case are sold as is and there are no returns. Only items shown in the photographs are included. Please feel free to ask questions and I will do my best to answer them, but recognize that I am by no means an expert on clocks.