Survey chronometer by Thomas Mercer Chronometers

Catalogue description — Royal Museums Greenwich

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. When in use, the chronograph mechanism feeds a paper tape, at a very even rate (up to about one inch per second is all that is needed), under two sharp pins side by side, suspended over it. These pins are each connected to an electrical solenoid, and can each be ‘pecked’ down onto the tape as it passes under, and prick a hole in it at a specific time. One of the pins is connected to the chronometer and marks a hole in the moving tape every second. The chronometer seconds wheel has one tooth removed, enabling a space at the 60th second, which then records the top of the minute on the tape (some survey chronometers, such as this example, incorporate a separate pair of ‘one-minute’ contacts as well). The other pin is connected to a simple electrical switch, or ‘key’, which the observer holds while viewing through the prismatic astrolabe, and with which he can mark the instant he observes the star reach the specified zenith distance. Once that observation (or series of observations) has been made, the tape which has been spooling out of the chronograph can be cut off and the first minute of the observation marked on the paper so that the successive minutes and seconds are identifiable. Then, working along the tape, the marks made by the second pin, recording the moment of the actual observations, can be measured along the series of seconds marks on the tape. The instant of the mark, between two seconds points, is then measured using a ruler, and provides an accuracy of 1/20 of a second.