Barometers Realm

Immediately beneath the bezel is another rather unusual structural element: a rotating brass ring painted black. This supports a substantial height-adjustable magnifying lens which can be moved freely around the circumference of the instrument. The lens can be positioned opposite any part of the scale and allows its extremely fine divisions to be read with confidence without bringing the instrument close to the eye. Such an arrangement is characteristic of precision surveying aneroids.

The inner surface of the aluminium case is coated with a matt dark-grey paint, most probably a stoving enamel or nitrocellulose enamel applied over a previously sand-blasted surface. This coating served two engineering purposes. First, it protected the aluminium from the damp atmosphere of mine workings, frequently laden with condensation and chemically aggressive impurities. Secondly, it electrically isolated the aluminium case from the numerous brass components of the mechanism, preventing the formation of galvanic couples and the electrochemical corrosion associated with them.

The dial deserves particular attention. It consists of two separate brass discs — an outer annular scale plate and a central circular disc — both coated with a thin matt cream-coloured enamel. This characteristic warm cream tone, combined with the very slight micro-texture of the surface, gives the instrument an exceptionally refined appearance while at the same time eliminating reflections that could interfere with precise reading.

The outer ring carries an exceptionally detailed barometric scale produced by fine etching or engraving, with the recessed markings subsequently filled with a black compound. The scale covers a range from 28 to 33 English inches of mercury, a range characteristic of mine-surveying aneroids of this type. The full range is distributed around the entire circumference of the dial, allowing each inch to be greatly expanded. Furthermore, every inch is subdivided into hundredths, or 0.01 inHg. The movable magnifying lens is provided precisely so that each of the one hundred divisions within a single inch can be read reliably.

The central disc bears the Casella London trade mark, the indication COMPENSATED, signifying temperature compensation of the mechanism, and the unusual instruction “TO BE READ HORIZONTALLY”. The latter is not merely a recommendation but an essential condition for obtaining the accuracy for which an instrument of this class was calibrated.

The entire kinematic system of the mechanism lies in the plane of the base of the aneroid capsule. When the instrument is placed horizontally, the weight of the capsule and of the lever system acts in a constant and reproducible manner relative to their working movement. The bearings and pivots are loaded consistently downwards, creating a stable gravitational preload that could be taken into account during factory calibration. Tilt the instrument, and the same gravitational forces acquire transverse components acting on the levers and pivots, introducing additional bending, friction and parasitic moments.

The significance of this becomes clear from a few figures. The complete scale range, from 28 to 33 inches of mercury, occupies one full revolution of the pointer and therefore corresponds to five inches of mercury. A single division of 0.01 inHg thus represents an angular movement of approximately 0.72° on an eight-inch dial. At the same time, the total working movement of the aneroid capsule over the entire pressure range is only about 0.8–1 mm, meaning that a single scale division corresponds to approximately two microns of membrane movement. The instrument must therefore reliably resolve movements on the order of a micron. Against such a background, any redistribution of the weight of the lever system caused by tilting the case is no longer negligible and can easily amount to several scale divisions. For this reason, readings are to be taken only with the instrument in the horizontal position.

Another conspicuous feature of the front is the anti-parallax mirror. Unlike most precision instruments, in which only a narrow mirrored strip is placed alongside the scale, here the mirror takes the form of a full-size brass disc approximately equal in diameter to the dial itself. Its front surface was nickel-plated and carefully polished to a mirror finish. The two brass components of the dial are mounted directly over this disc, while the broad exposed area between them remains reflective and performs the anti-parallax function. By aligning the reflection of the pointer precisely with the pointer itself, the observer ensures that the eye is positioned exactly normal to the scale and eliminates any error caused by viewing angle.

The japanned indicating pointer likewise emphasises the instrument’s status as a precision measuring device. It is exceptionally long and slender, made of steel and coated with black lacquer, providing a combination of high rigidity, low mass and excellent contrast against the light-coloured dial.

At the heart of the measuring mechanism is a large corrugated aneroid capsule made of beryllium bronze. It differs from conventional aneroid capsules above all in its geometry. Instead of two nearly flat corrugated diaphragms joined around their rim, it employs two shells of revolution with pronounced initial convexity, spherical or slightly conical in form. As a result, the capsule acquires a characteristic vase-like profile. This should not be confused with the so-called “Dent vase”, a term used for the shape of the internal cavity of an evacuated aneroid capsule. Here, the height of the external “vase” is created specifically by the convexity of the diaphragms rather than by the thickness of the rim joint or by the depth of a stacked capsule assembly.

This geometry offers a major engineering advantage. With an eight-inch dial, as already noted, the tip of the pointer travels almost half a metre along the scale between 28 and 33 inches, so that a single division of 0.01 inHg corresponds to roughly one millimetre of movement at the pointer tip. If a conventional flat capsule with a total working movement of only three or four tenths of a millimetre were used, a mechanical amplification ratio on the order of fifteen hundred would be required. Along with the useful signal, friction, backlash and lever flexure would then be magnified to a comparable degree. By using a deep convex capsule with a working movement of about one millimetre, the required amplification can be reduced to only a few hundred, correspondingly reducing the influence of mechanical imperfections.

The large effective diameter of the diaphragm provides an active area on the order of ten square centimetres, so that changes in atmospheric pressure generate corresponding changes in force measured in kilogram-force. Against forces of this magnitude, the resistance of the lever train becomes virtually negligible, and the threshold of sensitivity is determined not primarily by friction in the bearings but by the elastic hysteresis of the beryllium bronze itself.

It is also noteworthy that the scale is linear around the entire circumference. This is achieved by balancing two opposing physical effects. On the one hand, as the corrugations deflect further, their sensitivity tends to decrease and the response curve becomes progressively flatter. On the other hand, the initial convexity of the diaphragms, when loaded from the convex side, produces an increasing sensitivity. By carefully selecting the depth of the corrugations and the degree of initial curvature, Casella’s engineers were able to make these effects largely cancel one another, producing an almost perfectly linear response throughout the working range.

The aneroid capsule is tensioned by a powerful steel C-shaped spring mounted on a substantial cast-iron crosspiece. This spring provides the necessary initial force and at the same time helps define the working characteristic of the entire measuring system. The main lever, connected to the free end of the spring, is made of brass and incorporates bimetallic temperature compensation, greatly reducing the effect of temperature changes on the indications.

A notable feature of the main lever is the presence of several adjustment holes for connection to the oblique link transmitting motion to the rotating angle lever of the main arbor. A similar series of holes is provided on the angle lever itself. This arrangement allowed the geometry of the transmission to be selected with great precision during factory adjustment, enabling the link to be placed in the optimum position to suit the individual characteristics of each particular mechanism.

The angle lever pivots on a trunnion-type mounting carried in precision brass supports. From its vertical arm, motion is transferred to an extension connected to a fusee chain. The latter rotates the pointer arbor, on which is mounted a fine spiral return spring or hairspring, maintaining constant tension in the chain and eliminating backlash in the pointer.

Taken as a whole, this Casella mine-surveying aneroid represents one of the most sophisticated examples of mechanical barometric instrumentation of the mid-twentieth century. Every detail — from the construction of the aneroid capsule and the geometry of the lever transmission to the arrangement of the dial, the anti-parallax system and the movable reading lens — is devoted to a single purpose: the most accurate possible measurement of atmospheric pressure. Instruments of this kind stood at the intersection of barometry, geodesy and precision mechanics, and they remain an excellent demonstration of the extraordinary level of engineering refinement achieved by British scientific-instrument makers in the period immediately preceding the widespread adoption of electronic measuring methods.