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Object Overview
This German Pocket baroscope of about 1900 is an aneroid barometer with a rotating differential pressure scale that displays departures of the pressure from a norm the owner sets himself. It is meant not simply for reading the pressure, but for seeing at a glance how far the pressure stands above or below the norm that has been set (±40 mmHg).
The case is barrel-shaped, turned from brass, with two grooves running round the band. The back cover is slightly domed and plain, without signature or marks; near its edge a small round hole is drilled to give access to the calibration screw of the movement. On top is a ball-shaped pendant with a loose ring for hanging the instrument on a chain.
At the front is a rotating bezel holding a flat silicate glass with a bevelled edge. Under 365 nm ultraviolet the glass gives off a soft greenish-yellow glow, most noticeable along the bevel and around the rim of the disc. Its source is the Mn²⁺ ion, added in small quantities to kill the green cast produced by iron impurities: manganese served as a decolouriser and at the same time endowed the glass with a soft greenish fluorescence under UV. Manganese was used as a decolourising agent until roughly the 1920s, when it was displaced by selenium; the change was driven by wartime shortages (WWI).
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aneroid capsule tensioned on a C-spring
brass, glass, nickel silver, silvered brass, steel
The dial is made of silvered brass and carries a concentric barometric scale of absolute pressure covering 66–80 cm Hg (660–800 mmHg). Below the centre is a two-line inscription: Baroscop and Made in Germany. The dial is secured by three blued screws.
The other, unusual scale sits on a separate ring, also of silvered brass, laid over the main dial; this ring is retained by the bezel, and the scale turns with it as the bezel is rotated. Instead of the altitude scale traditional for pocket models, what is used here is a relative scale of the departure of atmospheric pressure from an accepted "normal" value, expressed in millimetres of mercury.
The values etched on the ring form the following scale:
−40 … NORMAL … +40 mmHg relative to the chosen reference point, in steps of 10 mmHg.
The geometry of the outer and inner scales is matched: 10 units of the outer scale correspond to roughly 1 cm Hg (10 mmHg) of the inner one — that is, the spacing between the adjacent numbers −40, −30, −20… equals the spacing of 1 cm Hg on the main barometric scale.
The pointer is made of blued steel, its tail carrying a crescent-shaped counterweight. It is read by its long end, which passes over both scales at once.
Suppose we wish to treat 760 mmHg = 76 cm Hg as normal. We turn the outer bezel so that the word NORMAL comes opposite 76 on the main scale.
After that the pointer shows two things at once: the current absolute pressure on the main scale, and its departure over the elapsed period from the normal value that has been set, on the outer scale.
The "normal" absolute pressure depends on the height of the place above sea level. 760 mmHg is the accepted normal pressure at sea level. If a man is, for instance, high enough up, his ordinary local pressure may be 730 or 720 mmHg. An ordinary barometer would then permanently show supposedly "low" pressure, although for that place it is perfectly normal.
The owner of this Baroscop can therefore set NORMAL opposite the pressure usual for his own location. If, for example, the normal pressure of the district is around 74 cmHg, he sets NORMAL → 74, and the instrument at once shows the departure from the local norm.
The outer scale turns an ordinary absolute barometer into a barometer of pressure anomaly relative to an arbitrarily chosen norm. The owner appoints the reference point himself — the conventional 760 mmHg, the local norm for his own altitude, or simply the reading taken at the beginning of an observation — and from then on reads not a number, but the size of the departure from it.
In the last case the bezel with its scale performs the function of a set hand, with the difference that instead of a second pointer merely marking the previous position, a whole graduated scale of change is used, giving the amount of the rise or fall in millimetres.
At the heart of the instrument is a small aneroid capsule of German silver, held under tension by a steel C-spring mounted on a brass bridge. A brass main lever, extending from the free end of the spring, acts through a link upon a rotating angle lever set on trunnion pivots; motion is carried to the pointer arbor from the vertical arm of the angle lever by a fusee chain.
The aneroid capsule with its spring and the whole train are mounted on a brass plate which has one peculiarity — it is not fixed to the case of the instrument in any way, no screws being provided for the purpose. Instead, three small tubes, internally threaded at their ends, rise vertically from various edges of the plate, and into them go the screws that hold the dial. The resulting sandwich simply drops into the case and is pressed down by the bezel. A most unusual solution, and hardly one calculated to promote stable working.
The inscription Baroscop on the main dial is the name the maker himself gave the instrument. For a pocket barometer it is not standard, and the choice was plainly influenced by that additional scale of the degree of departure of the pressure.
The word appeared in the 1660s in the circle of the Royal Society in London, from the Greek βάρος, "weight", and σκοπεῖν, "to observe". "Baroscope" is credited to Robert Hooke, "barometer" to Robert Boyle; both terms came into use practically simultaneously and at first denoted one and the same device — the Torricellian tube. They differed only in their internal logic: one word spoke of measuring, the other of observing.
Later, by the nineteenth century, different meanings had settled upon them. A baroscope came to mean an instrument that shows changes of pressure without measuring it exactly; the same word also denoted the classroom balance with a hollow sphere used to demonstrate the buoyancy of air. It is in precisely this technical sense that the inscription on the dial is to be read: not as a trade name, but as an indication of the character of the instrument — to watch, not to measure.
The maker's name is absent from the instrument. The English marking Made in Germany points to an export destination and places the piece after the British Merchandise Marks Act of 1887, which made the indication of country of origin compulsory; the styling of the case, the dial and the lettering allows the dating to be narrowed to the turn of the nineteenth and twentieth centuries.
Pocket aneroids with a rotating bezel are common enough, but their bezel almost always carries an altitude scale for the traveller and the alpinist. Here the maker did otherwise and brought out not height but the anomaly of pressure, turning a travelling trinket into an instrument for watching the weather. No analogue has been found in the reference literature on aneroids, in known museum collections, or on the antiques market: the idea, it seems, called for an owner who understood what a barometric tendency is, and for that reason it never took root — which makes the surviving example all the more valuable.