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Object Overview
Before us is a Pocket Engineering Barometer-Altimeter (Orometric Barometer), made approximately 1895–1915, in which the atmospheric-pressure range was deliberately subordinated primarily to the altimetric function. The instrument bears serial number 886, marked directly on the face of the dial beneath the company name. There is virtually no reason to regard this number as a production or assembly marking: a separate assembly number 203, intended for internal use during manufacture and assembly, is engraved inside the instrument case and also on the reverse of the dial plate. Thus, 886 should be regarded as the serial number of the completed instrument; its low value is one of the significant arguments in favour of a comparatively early date for this example.
886
aneroid capsule tensioned on a C-spring
brass, glass, leather, nickel silver, nickel-plated brass, silvered brass, steel
The instrument was made for and sold by La Filotecnica, Milan; the origin of the internal aneroid movement has not yet been established.
In contemporary instrument terminology, this type was known as an orometric barometer (French baromètre orométrique), a term used for precision barometers specifically intended for determining altitude and differences in elevation from atmospheric pressure. Historical instrument catalogues also used orométrique and altimétrique as essentially synonymous terms.
This is not an ordinary pocket meteorological barometer. The instrument was intended primarily for determining differences in elevation from changes in atmospheric pressure and could be used for engineering, surveying, topographical and mining work. Such a purpose corresponds perfectly with the profile of La Filotecnica: the firm manufactured theodolites, tacheometers, levels and other surveying and measuring instruments, while its specialised catalogues were addressed directly to ingegneri e agrimensori — engineers and surveyors. La Filotecnica catalogue No. 137, dating from approximately 1912, brings together geodetic, topographical and meteorological instruments, including barometric instruments.
Barometric levelling made it possible to determine the difference in elevation between two points without direct geometric levelling: as altitude increases, atmospheric pressure decreases, and a sufficiently sensitive aneroid makes it possible to relate the magnitude of this change to the difference in elevation. The absolute accuracy of this method depends on atmospheric conditions, temperature and the simultaneity of observations, but its great advantage lay in its speed and portability. A small pocket instrument could be used where carrying full-sized surveying equipment was inconvenient or where a rapid approximate determination of altitude was required.
The instrument is housed in a compact, round nickel-plated brass case, resembling a large pocket watch in form. The back is slightly convex and contains two screws securing the internal movement, as well as an aperture providing access for adjustment of the instrument without dismantling the case.
At the top is a spherical pendant with a small, freely moving suspension ring. This allowed the instrument to be carried on a cord, chain or strap, or to be secured during field measurements.
The front is closed by a broad rotating knurled bezel. Fine knurling around its entire circumference provides a secure grip for the fingers. The bezel holds a flat mineral-glass crystal with a bevelled edge. Attached to the glass is a small convex magnifying lens, or “bubble”, which can be moved around the circumference of the scale by rotating the bezel. It acts as a local magnifier, substantially enlarging a selected small section of the finely divided scale and thus facilitating precise readings.
On the inner side of the bezel is a separate silvered-brass ring carrying the altitude scale. Consequently, when the bezel is rotated relative to the stationary dial, not only the magnifying lens but also the altitude scale itself moves. This arrangement allows an initial altitude or initial pressure to be set before measuring a difference in elevation.
The non-linear altitude scale is graduated in metres and extends to 4000 m. Its divisions are deliberately spaced unevenly. This is neither a defect nor a decorative feature, but a direct consequence of the physical relationship between atmospheric pressure and altitude: atmospheric pressure decreases non-linearly with increasing altitude, so an equal change in pressure at different elevations corresponds to a different change in altitude.
It is this scale that most clearly reveals the purpose of the instrument. Unlike an ordinary barometer, in which atmospheric pressure is itself the final quantity being measured, here pressure also serves as the means by which altitude is determined.
The main dial consists of a circular silvered-brass plate. It carries a finely divided concentric barometric scale in centimetres of mercury, extending from 46 to 76 cm Hg. At the centre are the inscriptions COMPENSATO, “LA FILOTECNICA”, MILANO, and the serial number 886.
The range of 46–76 cm Hg is particularly revealing. Conventional meteorological barometers intended for use near sea level generally have an upper limit of around 78–80 cm Hg, enabling them to register periods of high atmospheric pressure. Here, however, the scale ends at 76 cm. This is not a limitation of the instrument, but a deliberate design choice: the barometer is optimised for measuring altitude and differences in elevation rather than for observing exceptionally high atmospheric pressure at sea level.
The relationship between the two scales becomes especially clear when their values are converted. The upper limit of 76 cm Hg corresponds to 1013.25 hPa, the standard atmospheric pressure at nominal sea level. The lower limit of 46 cm Hg is approximately 613 hPa and, in the standard atmosphere, corresponds to an altitude of roughly 4040 m.
Thus, the two ranges correspond almost perfectly:
In other words, La Filotecnica effectively used almost a complete revolution of the movement to represent a vertical range extending from sea level to approximately 4000 metres. This allows a substantial portion of the dial circumference to be devoted to the pressure range actually relevant to altimetric measurements.
Such an arrangement was particularly well suited to terrain involving substantial differences in elevation. In addition to ordinary engineering and surveying work, the instrument could have been used for topographical surveys in mountainous regions, exploration and mining work, mountaineering and military topography. Geographically, such a range was particularly appropriate to Italy, with its Alps and Apennines. A potential user might have been an engineer, land surveyor, geodesist, mining specialist, military topographer or traveller working in mountainous terrain. Theoretically, its range would also have permitted the instrument to be used in early aviation.
Inside is a compact conventional aneroid movement, mounted on a brass plate. Its pressure-sensitive element is a small circular corrugated aneroid capsule made of nickel silver. This hermetically sealed, partially evacuated capsule deforms in response to changes in external atmospheric pressure: as the pressure decreases it expands, and as the pressure increases it contracts.
The upper diaphragm of the aneroid capsule is tensioned by a powerful C-shaped steel spring mounted above it on a brass crosspiece. The system acts rather like a balance, counteracting the column of atmospheric air pressing upon it and forming a sensitive measuring element that responds to changes in pressure and transmits them to the pointer arbor through a system of levers.
The main lever incorporates temperature compensation. This is achieved by means of a stable bimetallic strip, which changes its geometry as the temperature varies and thereby partially compensates for temperature-induced errors in the elastic system. The presence of this compensation is explicitly indicated by the word COMPENSATO on the dial.
The main lever is connected by a fine link to the bell-crank lever of the main arbor. From the vertical arm of this bell crank runs a fine fusee chain, which transmits the movement to the pointer arbor. The chain transmission converts the very small linear displacement of the sensitive element into a considerably larger angular movement of the pointer, while at the same time minimising backlash and friction. The restoring force of the pointer system is provided by a fine spiral hairspring.
The movement is extremely compact: the sensitive capsule occupies almost the entire central portion of the plate, while the system of levers, chain transmission, temperature compensator and pointer mechanism are arranged around it. Despite the instrument’s small dimensions, its construction constitutes a fully developed aneroid mechanism specifically adapted for field use.
The barometer survives with its original, relatively rigid moulded dark-brown leather carrying case. Its circular lower section fits closely around the body of the instrument while leaving the dial exposed for observation. The hinged cover is secured by a strap with a press-stud fastener. This arrangement allowed the barometer to be carried protected from impacts and dirt while still permitting it to be removed quickly whenever measurements were required.
This instrument occupies an intermediate position between a pocket aneroid barometer and a specialised surveying altimeter. Its most characteristic feature is not simply the presence of an additional altitude scale, but the coordination of the entire measuring system with an altitude range of approximately 0–4000 m. The unusual 46–76 cm Hg barometric scale, the non-linear adjustable altitude scale, the magnifying lens for precise readings, the temperature-compensated movement and the compact protected case together form a single specialised instrument.