Aneroid barometer having silvered and engraved 6” dial calibrated in inches of mercury with a range from 28 – 31, finely divided to 100ths”. Standard meteorological terms, the upper part marked “Compensated” (for temperature), the lower signed by the retailer “C. Baker, 244 High Holborn, London.” Fine blued steel pointer, brass vernier supported on twin stays and operated from outer glass-mounted frame.

Fine standard configuration movement driven from twin 2¾” capsules tensioned on a C spring. The whole contained within a lacquered brass O-G case marked internally “1603,” extension post and suspension ring, all supported on a carved oak stand.

Condition: This instrument has been the subject of extensive conservation, servicing and calibration. The dial excellent, circumferential shadow, the vernier slightly out of true but perfectly operable. The brass case with traces of original finish, extensive losses. The wooden stand sound and with good colour. As confirmed in the laboratory test, this instrument is still capable of remarkable accuracy, less than 0.8” out across the range, see performance chart.

Comments: This is a very rare instrument – I think I have noted one other but many years ago. Manufactured by Negretti & Zambra, this is a serious attempt at approaching ultimate accuracy with an aneroid barometer – the provision of twin capsules, unusual in itself, suggests a movement designed at the outset to be accurate, the setting of the arbor and associated hairspring are additional clues.  The finely divided dial, vernier and fine pointer complete the whole as a very high quality instrument designed for specialist applications.

Charles Baker, a maker and retailer of scientific instruments, was based at 244 High Holborn, London until 1909.

A very similar twin-capsule movement is found in what is acknowledged to be the oldest known production aircraft altimeter, manufactured by Negretti & Zambra c1911.

Very much a collector’s piece.