Antique barometer having white card 4½” dial annotated “Stormy, Much Rain, Change, Fair, Set Fair, Very Dry,” calibrated from 25-31 inches of mercury. The lower portion marked “Aneroid Barometer” and serial no “22720.” Blued-steel pointer and gilt brass telltale under flat glass with fine knurled gilt brass telltale adjustment wheel to centre. Lacquered brass flared drum form case, the bezel with trademark Dent engine-turned herringbone pattern. 2½” capsule-driven, substantially developed and innovative movement with static mass lever and low friction cone-mounted ratio compensation. Set pillar with suspension ring to top. The reverse with screw calibration port set at 3 o’clock as viewed.
The whole contained within its contemporary solid mahogany carved-out case with hinged lid and circular viewing port, secured with a brass hook and eye.
The instrument movement having been conserved and serviced, the dial with minor losses, the brass case with abrasions, some denting, areas of oxidation and general wear at contact points with box, retaining a good proportion of original lacquer.
The movement in this Dent barometer is extremely unusual – Collins, Phillip, Aneroid Barometers and Their Restoration, p16, fig 2.10, shows this same movement from a similar Dent barometer of the same period.
Makers of these instruments at this time were aware of a number of shortcomings in the basic aneroid design: principal was the lack of linearity and a phenomenon known as hysteresis, endemic in the pressure sensing capsule. The mechanical connections within the movement resulting in hang up caused by static friction.
Dent, the genius innovator and mechanical engineer, attempted to address these issues. Lucien Vidi’s original design essentially entailed five pivot points in order to transmit and amplify the signal from the capsule to the head of the fusee chain connected to the arbour. Dent set out to substantially reduce the issue of friction, by changing the angles and positions of the interconnecting rods and levers and by causing them to interconnect at points, rather as a compass needle on a point. By changing the orientation of the cross shaft, placing it vertically from the horizontal, there became two principal bearing points of contact (from the original five), and with the instrument lying on its back as in a deck case, this essentially became one, that at the base of the now vertical cross shaft – the top also on a pivot might be largely discounted as this served only as a steady.
A further measure to address the issue of hang up was the fitting of a static mass lever, essentially a large weight mounted on a rod and extending in front of the C-spring. The rod is secured to a substantial cross shaft transversely mounted to the C-spring, connected to and directly above the centre of the capsule. The instrument would be affected when subjected to any vibration, including speech. These vibrations transmitted into the movement would cause it to oscillate as a result – the static mass being less affected, the difference in amplitude of these oscillations causing any remaining static friction to be released.
This is a fascinating study in innovation and development of the aneroid system, encompassing some of the most significant major changes to the original design to date. Without doubt the work of one of the world’s leading horological and instrument technicians.
Only three of these instruments have been identified to date: serial nos 22175, 22720 and 22769. A narrow band in manufacturing terms, it is not known if the production was consecutively numbered – however this shows a dispersion of some 600 instruments assuming 22175 is correctly read.
The pressure test graph below shows remarkably good performance given the early date of manufacture and certainly a vindication of Dent’s masterful innovation. Progression is smooth and stepless with very good resolution (sensitivity).
A collector’s item without doubt.
Dimensions: 5″ wide x 7″ deep x 2″ high
Stock No: BA0917
Price: Vavasseur Archive – not currently for sale