British military issue precision isothermal altimeter by Negretti & Zambra, London, no. P/113129 c1955

Aneroid altimeter having 4¾” off white enamelled dial calibrated in metres of altitude and finely divided to 5m, the upper part marked “Height,” “Metres” and “Isothermal (50°F) Metres, H=19075.5 Log (29.9/P),” the lower signed by the maker “Negretti & Zambra, London,” and serial no. “P/113129.”

Straight thermometer calibrated in degrees Fahrenheit  with a range from 0-120°. Light weight tubular aluminium knife pointer, bevelled glass set within a rotating bezel.

The movement, marked “15,” arranged between two plates with thermal expansion slots, driven from twin sylphons acting in opposition about a common fulcrum, rotation afforded on spring steel, the arbor and primary lever set in fine pivots, the signal transmitted via a chain to a fusee. All located within the case on a machined lip and retained on chassis-mounted plates running in a continuous machined groove affording rotation for correcting on a rack and pinion.

The whole mounted in a substantial nickel brass drum form case, the verso marked “Service Part No W.10/WK.00152,” broad arrow government military property mark. Together with its original serially numbered heavy foam-lined aluminium transport case with lock down lid, the top marked “P/113129,” and “ Service Part No W.10/WK.00152,” inner lid marked “19,” cast in webbing strap loops.

Condition: The subject of a full service, conservation, and calibration under laboratory conditions. The movement working well across the range, see performance chart. The instrument generally in very good condition, the dial clean and clear, glass undamaged. The aluminium case retaining much original grey painted finish, some chipping and abrasions some losses to base. The bezel fitted loupe and the case inner lid mounted clip and small adjustment screwdriver missing.

Comments: These instruments were the precursor to the later electro-mechanical types introduced by Mechanism of Croydon. A highly developed and technically fascinating manifestation of the aneroid barometer, some of the innovations include: matched sylphons or pressure cells which reduced in pressure as a pair after assembly of the movement (note the long tails emanating from each cell); the inclusion of expansion slots to reduce the effects of thermal expansion of the movement plates; a super lightweight pointer to help reduce static friction in the arbor pivots. In short, this is a masterpiece in engineering inspired by science.

Although this is serial number P/113129, these serial numbers relate to all instruments made by Negretti & Zambra relating to pressure from 1921 onwards. It is likely that these instruments were made in very small numbers reflected by the few that appear on the market. In addition, the movement is marked “15” and the case “19.” 

These instruments are occasionally seen. They are under-researched and substantially undervalued – of the innovations on the basic Vidi movement introduced over 100 years before, this is one of the most important.