Experimental aneroid barometer having 3½” silvered and engraved dial with a barometric range 24–31 inches of mercury, the upper portion marked “Experimental Aneroid Barometer,” the lower part marked “By Negretti & Zambra, London” with the maker’s logogram. Fine blued-steel pointer set beneath a plain glass retained in a threaded screw-down bezel with ‘O’ seal. Substantial 2½” deep drum case set with counter-sunk screws, compensation port to rear with the aperture at 10 o’clock. The extremely unusual movement mounted on opposing sides of a rotatable brass circular chassis plate driven by a four-disc sylphon-type capsule with single evacuation tube. Single spindle raised on twin-turned columns and braced with blued flat steel beam and turning on tapered end pivots, attached leaf spring (primary lever) with range adjustment coupled directly to capsule top, driven curved lever with longitudinal position setting, inertial static relief mass, and fusee chain to arbour. Chassis-affixed rack engaged with compensation pinion on a short axially-mounted shaft causing the movement to rotate within the case and activated by a slotted head to the case rear accessible through compensation aperture.

The dial with silvering lacking, some marks, the case with wear to chromium finish, pinpoints of oxidation. ‘O’ seal to glass replaced. Very sensitive and highly accurate with near perfect progression. Overall condition fair.

By any standards, a huge and innovative departure from the generally accepted aneroid mechanism and from Vidi’s original pattern. Principally, it effectively eliminates one pivot and determines a more direct connection of the capsule or sylphon to the pointer, allowing reduced friction and error, and thus greater sensitivity and accuracy.  The capsule is also innovative, formed as it is from tempered and hardened steel.  Finally, temperature compensation in this instrument would appear to be based on a Torricellian vacuum within the capsule.

This antique experimental aneroid barometer was not a sale item. It was manufactured expressly by Negretti & Zambra for their own research and development department and was the precursor leading to the development of later highly accurate devices that would eventually measure pressure changes to three decimal places. Production costs for this instrument would have been high and its design would have been a closely guarded secret.

So what have we got? A unique item? Well that’s hard to say: extremely unusual, yes, highly significant in the evolution of the aneroid principle, yes, and in addition by one of London’s top makers.  This is a piece for the serious collector.