Barometers Realm

What is especially remarkable is that this demonstration instrument does not resemble a temporary laboratory model at all. On the contrary, it presents itself as a fully finished, almost display-quality object. The polished wooden base, glass dome, exposed mechanism, and large scale create an instrument designed not merely for function, but also for spectacle. This is highly characteristic of German educational apparatus from the late nineteenth and early twentieth centuries: such devices were intended simultaneously to explain, to persuade, and to impress.

The historical reference work Sound and Science notes that in 1876 Max Hans Robert Kohl founded a workshop for precision mechanics in Chemnitz, after which the firm gradually developed into one of Germany’s largest manufacturers of scientific and technical apparatus. The company specialised in producing systems and devices for technical and scientific purposes and supplied laboratories, universities, and schools throughout the world with an ассортимент reaching approximately 4,000 different instruments. Max Kohl was therefore widely recognised as a major producer of educational physical apparatus. The firm operated according to a hybrid industrial model: it designed, manufactured, assembled, and distributed an enormous range of laboratory equipment. Some instruments were entirely proprietary designs, others were manufactured according to established scientific patterns, while certain components may have been sourced or assembled from specialised suppliers.

Under catalogue number 52.796, the present instrument is described by Max Kohl as: “Demonstration Aneroid Barometer for suction, with rubber tubing and mouthpiece, covered by glass bell, can be used horizontally and vertically.” This description itself is extremely important. It emphasises that the instrument was intended both for tabletop and wall-mounted use. In other words, this is not simply “a barometer under a dome,” but rather a complete miniature pneumatic demonstration system.

Indeed, the instrument consists of a thick mineral-glass dome fitted as tightly as possible onto a brass base containing the aneroid mechanism. The entire construction is mounted upon an elegant wooden stand approximately 160 mm in diameter, painted and lacquered in imitation ebony. The base rests upon two pointed feet, while the role of the third foot is simultaneously performed by a brass screw with a brass suspension eyelet. This detail is particularly ingenious: when used horizontally, the eyelet functions as a third support point; when mounted vertically, it becomes a hanging loop for wall suspension — precisely as stated in the catalogue description. It is an exceptionally simple, practical, and elegant engineering solution executed to a remarkably high standard.

The dial, measuring approximately 120 mm in diameter, is made of cardboard and formed as a thin curved arc in order to preserve the greatest possible visibility of the mechanism from all sides. The cardboard scale is mounted upon a thin brass backing plate supported by two brass pillars. The barometric scale is calibrated in centimetres of mercury over the range of 72–80 cm and bears Dutch weather indications: Storm, Regen of Wind, Veranderlyk, Mooi Weer, and Bestendig. The inscriptions are executed in capital serif lettering, with certain terms differentiated by varying stylistic treatments, lending additional decorative expressiveness to the scale.

The instrument operates by means of a conventional aneroid movement centred around a substantial 70 mm aneroid capsule made of nickel silver. The capsule is tensioned on a C-shaped steel spring fixed to a cast-iron support bridge mounted upon two pillars.

The movement of the aneroid capsule wall, caused either by atmospheric pressure or by artificially increasing the air pressure beneath the glass dome, is transmitted through the C-shaped spring to the principal brass lever, and from there via a connecting rod to the knee lever of the main arbor. The knee lever is mounted upon two spherical supports and rotates through a conventional pivot connection; it is additionally fitted with a spherical counterweight.

The motion is then transferred from the knee lever to the pointer arbor by means of a fusee chain. This chain rotates the pointer shaft, while a counteracting spiral spring wound around the pointer arbor maintains constant chain tension and returns the pointer to its original position.

The indicating hand is made of steel with heat bluing, which serves both protective and decorative purposes. As a result, the pointer acquires a deep blue coloration sharply contrasting with the pale dial and brass mechanism.

Originally, the instrument was supplied with an India-rubber pressure bulb for demonstrations; however, the original bulb has not survived, and the instrument is now fitted with an ordinary mid-twentieth-century rubber bulb. This bulb is connected to the brass mouthpiece through a rubber tube. The mouthpiece itself communicates with the internal space beneath the glass dome through an inlet adapter featuring concentric circular grooves — a construction well known from Bourdon pressure gauges.

The principal idea behind such a demonstration instrument is extremely simple. An ordinary aneroid barometer responds to natural atmospheric pressure changes far too slowly and subtly for an effective lecture demonstration. Therefore, in the demonstration version, the mechanism was placed beneath a glass bell or within an open structure connected to a rubber tube, bulb, mouthpiece, or pump. The demonstrator could slightly compress or rarefy the air, and the pointer would immediately begin to move.

In this way, the demonstration barometer solved an important pedagogical problem: in a conventional instrument the observer sees only the dial and the pointer, while the actual operating principle remains concealed inside the case. Here, however, the mechanism is entirely exposed and transformed into a visible “living diagram.”

Under normal conditions, the aneroid capsule is subjected to atmospheric pressure. The air inside the capsule has been removed or greatly rarefied, so the external pressure constantly attempts to compress its corrugated walls. When the external pressure increases, the capsule flattens slightly; when the pressure decreases, it expands again under the action of its own elasticity and spring system. These extremely small movements are greatly amplified by the lever transmission and converted into visible pointer motion.

In the educational instrument, instead of waiting for an actual change in weather, the user artificially creates the effect. If air is pumped beneath the glass dome, the internal pressure increases, the capsule compresses, and the pointer moves toward higher pressure. If air is withdrawn, the pressure decreases, the capsule expands, and the pointer moves in the opposite direction. In this way, anyone wishing to understand the principle of the aneroid literally sees that the movement of the barometer pointer results from the deformation of a metallic vacuum box. At the same time, the instrument also performs its intended function of measuring current atmospheric pressure, since the space beneath the dome communicates with the surrounding atmosphere through the mouthpiece.

The demonstration aneroid explains several principles simultaneously. First, it shows that the aneroid reacts not to “weather” as an abstract phenomenon, but to the pressure difference between the external environment and the rarefied interior of the capsule. Second, it makes the amplification of motion visible: the capsule itself moves only imperceptibly, yet the lever system and pointer mechanism transform this microscopic movement into a large and clearly visible indication. Third, the instrument demonstrates the reversibility of the process. Increase the pressure — the pointer moves one way; decrease the pressure — the pointer returns. For the classroom this was ideal: a single instrument could demonstrate within a minute what in nature might occur over hours or even days.

The value of a demonstration barometer exists on several levels simultaneously. The first level is technical. Such an instrument reveals the aneroid mechanism in open form: the large capsule, lever transmission, fusee chain, and pointer mechanism are all fully visible. The second level is pedagogical. It was created not merely to measure pressure, but to explain a physical process. This is not simply a domestic meteorological instrument, but an apparatus of visual proof. The third level is historical and scientific. It demonstrates how physics was taught at the turn of the nineteenth and twentieth centuries: through beautiful, expensive, materially persuasive apparatus. In the era before screens, animation, and digital simulations, such devices functioned as genuine “living diagrams” of physical laws. Finally, there is the aesthetic level. The black polished base, glass dome, exposed brass mechanism, and large scale create an object that simultaneously functions as a barometer, an educational model, and a kind of display sculpture of a physical principle.

This Demonstration Barometer and related instruments represent a very special branch in the history of aneroids — neither domestic, marine, nor altimetric, but specifically educational and demonstrational. Their purpose was to make the invisible visible: to show how changing air pressure deforms a metallic vacuum capsule and how this microscopic movement is transformed through levers and chains into the motion of a pointer. In this sense, such instruments occupy a unique position between physical experiment, precision engineering, and the decorative scientific art of the early twentieth century.