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Watts (Rank Precision Industries, Leicester, U.K.


Autoplumb ST555-2

Serial Number



235 x 180x 260 mm



The "Autoplumb" is an early automatic zenith plummet. Yje main body of the instrument carries a label "Watts". The plaque with the serial number mentions "Rank Precision Industries Ltd, Leicester, U.K." (and the type number ST552-2).

Thecover of the objective lens on top as well as the wooden box are marked "Hilger & Watts, 98 St. Pancras Way, London". The instrument has a special detacheable tribrach which allows to shift the instrument sideways by about 28 mm (for centring above ground marks) without disturbing the levelling of the instrument. This design is sometimes referred to as "centring above the footscrews". The tribrach has a thread of abouit 85 mm diameter (3 inch 3/8, 12 turns per ich) for attachment on a fitting tripod.

The Autoplumb has a low power telescop for sighting downwards and a high power telescope to point upwards. The upward telescope has an optical compensator that keeps the line of sight vertical once coarse levelling has been carried out. The vertical telescope has a micrometer that allows to measure the 'errors' in verticality of marks above the instrument. (The line of sight is vertical when the micrometer reads 10.00. 1 revolution = 1/1000. Multiply with distance to get offset from vertical in length units.)

The precision is 0.3 mm/40 m (1:130'000) by measurement in two faces (180 degrees rotation about vertical axis between measurements). The instrument has four v-slots in the base that allow to lock the in instrument in four cardinal directions. There is a circular bubble (20') on top and a 20"/2 mm plate level for levelling.

Telescope: 30x magnification, minimum facus 1.8 m.

Reticule: horizontal line and 3 concentric circles

Compensator: Range 15', repeatability 1".

Downward telescope: focus 0.9 to 9 m, 17x magnification, double cross lines (one minute of arc separation), objective diameter 25 mm.

Weight of Instrument and wooden transport box: about 11.5 kg

The items include a tmatching tripod with telescopic legs.

History & comments



In wooden box. Instrument and tribrach in different compartments. 405 x 225 x 225 mm


Wear and tear on box, some paint damage on instrument. some corrosion on aluminium parts.


Catalogue No. ST552.

The instrument and matching tripod were donated by Bill Rein, Surveyor, Balgowlah NSW in April 2012.


Catalogued on 16 September 2014 by JMR. Photo by JMR.

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