Precision surveying barometer having engraved and silvered dial with a barometric range from 24–38 inches of mercury, the upper portion marked “Precision Aneroid,” “inHg,” serial no. “P/111671,” and maker’s signature “Negretti & Zambra, London” surmounting “NZ” logo, the lower portion further marked “Made in England,” inset micrometer dial calibrated to 1/1000th inches of mercury. Heavy crystal glass contained within substantial bezel. High precision silvered bronze movement constructed upon a substantial circular chassis marked no. “109” with expansion relief, opposing twin 2″ hardened steel capsules acting upon a common beam swinging about a leaf spring driving an intermediate spindle connected to the main lever. The top plate numbered “109” raised on four pillars acting as a capsule clamp and arbour platform. The arbour with helically-cut chain guide and flat section bronze return spring. Micrometer adjustment mechanism set below the dial within a sub-assembly. Contained within a massive silvered bronze drum form case, the rear with three transport case locating lugs, calibration adjustment port.
The whole contained within a cylindrical hard formed leather transport case having rotating glazed top viewing port with scale magnifier set within a heavy silvered bronze bezel. Hard formed leather protecting lid with hold down straps, shoulder strap.
The instrument, serviced and checked, in excellent working order; both instrument and top glass with some cosmetic scratches, compound magnifier with a flaw to one lens, the leather case in generally very sound order, complete with carrying strap.
This aneroid surveying barometer, designed to measure barometric pressure to 1/1,000th of an inch, and able to measure heights up to 5000ft and depths to 7000ft below sea level, has been constructed to the very highest standards of any contemporary scientific instrument. A number of notable features are evident: the long evacuation tails on the capsules suggest fine tuning of the vacuum whilst the instrument is tested, a feature only seen on one other instrument, the “Experimental Barometer” also showcased on this site that pre-dates this instrument and surely formed part of the basic research and design that is manifested in this very fine piece of equipment. (Another feature these two instruments have in common is the mechanical means by which fine adjustment is made for calibration.) Other notable features include expansion slots in the main chassis, opposing twin capsule design and, most obviously, the provision of the inset micrometer dial enabling extremely accurate readings to be taken.
Although mention of similar precision surveying barometers has been made in Negretti & Zambra’s catalogue of 1936, no other examples have been seen or located. It is most probable that these instruments were constructed in very small numbers and would have been extremely expensive, the end users possibly being organisations such as the Ordnance Survey, mining and civil construction companies.
Infinitely fascinating, this instrument is a study in design and engineering excellence – as an example of what is essentially an aneroid barometer, it is unsurpassed.
Dimensions: 7″ diameter x 6″ high
Stock No: BA0429
Price: Vavasseur Private Collection