A good five glass light oak cased aneroid Precision or Micro-Barograph serial no. R/12928 c1935

Micro-barograph comprising twin sets of spring steel mounted sylphons within a three-column cage, acting in opposition on a common spring steel mounted swinging bar with micro calibration adjustment and course setting. The hinged pen arm with jewel mounted spindle, further micro calibration screw.

All raised over a Damascus patterned full-length chassis engraved “Micro-Barograph, Negretti & Zambra, London,” with N&Z logogram, serial no. “R/12928,” and “Made In England.” A further plate marked “Precision Recording Barometer, Range 28/31” Hg. Pen Travel 5” for 3” Hg.” Ink station with bottle and dipper, arrestor bar with high and low stops. The 5” diameter x 6¼” high recording drum with captive key, sprung chart clip and lid.

Presented in a light oak case with hinged cover, and five bevelled glasses.

Condition: The subject of a major mechanical re-build, conservation and re-calibration under laboratory conditions, one sylphon changed for an identical original.

The mechanism with minor marks and signs of ageing, the chassis essentially clean and crisp. The oak case with some losses to lacquered finish but having a good overall shine and patina. The glasses damage free.

Comments: Not far off an antique at nearly a hundred years old, and yet this instrument would look perfectly at home in any modern day laboratory. Negretti & Zambra really scored here with a brand new design in pressure translation, an arrangement that closely follows that of their precision aneroid barometers of the time. This system appears to have been in production well up into the 1950s and possibly beyond, and certainly there would have been an overlap with the Mechanism and then Negretti Aviation manufactured Precision Electro-mechanical Aneroids first appearing in the early 1960s.

As for production numbers, this remains a conundrum: applying a little knowledge with a dash of common sense, I suspect actual numbers built were rather less than 500, and possibly as few as 200. The instrument, stock nos M2071/2, appears in Negretti & Zambra catalogues M2, M3, and M4, priced in the M2 catalogue, c1932, at around £40, and in the M4, c1950, almost £100. The “R” serial number applies to all N&Z production of recording pressure sensing instruments. So the serial number of this instrument is interspersed with the making of many other types.

The designation “Micro-Barograph” and “Precision Barograph” (or Precision Recording Barometer) can be confusing – essentially they are identical instruments, and only vary by the amount of amplification of the signal from the sylphons. This can be easily changed by relocating one pin in the movement, thus switching the instrument between Micro-Barograph and Precision.

The Micro-Barograph translates 1” Hg onto a vertical scale of 5”, the Precision Recording Barometer or Barograph translates 3” Hg onto a vertical scale of 5”. This instrument left the factory as a Micro-Barograph, evidently later re-calibrated to operate as a Precision Barograph.

Interestingly there is a catalogue price difference between the two, the Micro-Barograph being a few pounds more expensive. That will be due to the extra attention given to the instrument during final set up and calibration.

This instrument scores when it comes to levels of sensitivity and resolution. Negretti & Zambra claims a figure of 0.02” as a Precision Recording Barometer – from my tests that seems entirely reasonable, possibly even conservative. This is a truly remarkable piece of engineering, aesthetically very interesting to look at, and fascinating to analyse micro pressure fluctuations through the window of this very precise marvel of early C20th engineering.