Pocket barometer having finely struck 2½” silvered brass dial with twin scales, the inner calibrated from 20″ to 31″ of mercury, the outer (rotating) calibrated from 0–12,000ft, fine blued steel pointer, rotating bezel, carrying the outer altitude scale with knurled outer ring. The 1¾” capsule driven special movement fitted with a capsule recovery spring housed in a spun case. Turned brass pillar with suspension ring. The verso with compensation port at 8 o’clock with sliding cover, and marked with “A.M.” surmounted by the Royal Crown, “Ref .No. 6B 114,” “Barometer Aneroid Pocket Mark I,” “T. Wheeler, London” and “No 134.” Contained in its original stitched brown leather case with maroon velvet lining and marked “A.I.D.95.V” within an oblong border.
This is no ordinary altimeter. Wheeler was obviously well aware of the shortcomings inherent within the Vidi capsule design – namely hysteresis, essentially the time delay involved with the return to shape of the capsule after expansion at altitude and a return to ST&P. In order to increase the accuracy of the instrument passing into lower altitudes, an additional recovery coil spring has been added to the primary lever. This is the only type of these instruments so encountered with this innovative device. In addition, the primary lever has had anti-vibration nuts added to the fulcrum pins. Interestingly these two unusual features are strong indicators of the intended use of the instrument, ie that it should be subjected to rapid descent after prolonged exposure to high altitude together with long periods of vibration. These instruments were made as part of an Air Ministry contract, (RAF) – since this is definitely an altimeter there is little doubt that it had an aeronautical application. The annotation A.I.D. may suggest an aid, perhaps to an aircraft cockpit altimeter – there are many permutations that may be made.
The brass case retaining virtually all original lacquer, some losses in the locality of the bow, the dial clean and crisp with good vibrancy, the glass with minor scratches and slight hazing. The leather case in sound condition with good colour, the base with three small screw holes suggesting this instrument might well have been permanently fixed to a navigator’s table or other aircraft part.
Dimensions: 2¾” diameter x 1½” deep
Stock No: PB0452