A very good RAF issue precision isothermal altimeter by Negretti & Zambra, London no. P/113153, Service Part No. W10/WK 00152 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/113153.” 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 constructed principally from Invar, stuck with No “39,” arranged between two plates with thermal expansion slots, driven from two pairs of 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 Invar drum form case, the verso marked “Service Part No W10/WK 00152,” broad arrow government military property mark. Together with its original serially numbered heavy foam-lined cast aluminium transport case with lock down lid, the top marked “P/113153,” and “Service Part No W10/WK 00152,” inner lid marked “72,” cast in webbing strap loops. Inspection and calibration cross check label dated 29-1-93, associated stores docket dated 15-1-93, further tape label marked “19 Squadron.”

Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement working well across the range and remaining presentably accurate.

The dial crisp and clean, very minor enamel hairline cracks to right hand end of thermometer aperture. The aluminium transport case with marks, abrasions and some impressions, particularly in the base. The scale magnifier missing – these were almost all removed at some point, reason unknown.

Comments: These instruments were constructed to the highest possible standards, manufactured principally from Invar, a nickel iron alloy with very low coefficient of expansion. The precursor to the later electro-mechanical types introduced by Mechanism of Croydon, and a highly developed and technically fascinating manifestation of the aneroid barometer, some of the innovations include: matched sylphons or pressure cells which were 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 much reduced effects of thermal expansion of the movement plates; a super lightweight pointer to help reduce static friction in the arbor pivots. Noteworthy is the fusee about which the transmission chain coils, a feature designed specifically to approach the non linear response from the sylphons. In short, this is a masterpiece in engineering inspired by science and then state of the art engineering.

Although this is serial number P/113153, these serial numbers relate to a wide range instruments made by Negretti & Zambra from 1921 onwards. It is likely that they were made in very small numbers reflected by the few that appear on the market. 

From the view point of the collector or just the connoisseur of well-engineered instruments, this is a really fine item and worthy of any collection.