A very unusual aluminium aneroid aircraft maximum height recorder for Short Brothers, Aeronautical Engineers, London & Sheppey, No 168 by Negretti & Zambra, c1914

Aircraft altimeter having 4⅜” three step dial, the outer bezel rotated altimetric scale (1st step) marked “Feet” with a range from 0 – 12,000ft and divided down to 100ft, case mounted bezel lock, (2nd step) with crown operated index and marked in red “Ascent” and “Descent” with corresponding arrows, the dial centre (3rd step) with barometric scale calibrated in inches of mercury with a range from 20” – 31” divided down to 1/10th. The upper part marked “Maximum Height Recorder,” further annotated “Short Brothers, Aeronautical Engineers, London & Sheppey” with serial no. “168” and “Compensated.” Stylised blued steel pointer, bevelled glass.

The special dedicated lacquered brass movement driven from a single 2¾” nickel alloy capsule tensioned on a C spring, the re-designed cock fitted with a secondary pivoted spindle supporting a sprung rocking pawl, the arbor fitted with a uni-directional fine toothed gear or ratchet, the mechanism very similar to that found in pedometers. Intermediate chassis mounted actuating lever. Movement with hand marked date “9/14.” The whole raised on a circular aluminium chassis with foundry mark “TW.”

All contained within an aluminium drum form case, externally fitted with twin mounting lugs, Ascent/Descent dial operating wheel, bezel locking wheel and Maximum Height re-set or pointer release lever, marked “M.” The verso with calibration port.

Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement clean and working exceptionally well, the maximum height mechanism engaging and re-setting correctly.

The dial clean and crisp, the glass with a minor chip, the aluminium case pitted overall.

Comments: This Maximum Height Recorder is detailed in Negretti & Zambra’s M catalogue c1915 in the section “Scientific Instruments for Aeronautical Purposes of All Kinds”, page A, catalogue no. 274, priced at £ 7 10  0 plus 10/- extra for carrying a scale above 10,000ft. Additionally, the construction and detailing of this instrument closely resembles the early Negretti & Zambra Aircraft Altimeter, and there can be no doubt that both originated from the same maker.

Serial no. 168 might suggest at least 168 of these instruments were constructed – this is the only surviving example currently known. Short Brothers were test flying several new types of aircraft at this time, and it was likely that this instrument was specially ordered by them from Negretti & Zambra for the test flying programme.

The maximum height recording mechanism may be set to either record maximum height or to operate as a straightforward altimeter, rising and falling with changes in altitude. With the case mounted lever set to the “M” position, maximum height is recorded. Re-setting the pointer is achieved by moving the lever fully away from the “M” position, the pointer immediately swinging back to its ambient pressure position. This movement is quite violent and not unlike that of the re-set for a stop watch. In view of the mass of the pointer and other associated parts, there is a not insubstantial amount of potential energy released into what is a delicate train. It is therefore likely that some failures occurred. It should also be noted that with the rapid advancement of aircraft performance, a ceiling of 12,000ft very rapidly rendered the instrument obsolete (the Negretti & Zambra M catalogue of c1915 offered a scale of up to 15,000ft).

This instrument is a very important part of the history of early aviation.