Barometers Realm

This version of the Paulin was intended primarily for engineering and surveying work. The instrument was used for preliminary topographical surveys and barometric levelling — determining differences in elevation and ground levels when surveying routes for roads and railways, selecting sites for dams and reservoirs, laying pipelines and canals, and carrying out geological, mining, and forestry surveys. More broadly, Paulin System instruments were also employed in mining, scientific expeditions, meteorology, mountaineering, and aviation. For field investigations, their principal advantage was that a single observer could rapidly obtain a succession of elevation readings without the bulky combination of levelling staff and optical instrument traditionally required for precise levelling work.

The altimeter is housed in a substantial moulded case made of thick reddish-brown leather. The lid is joined to the base by a leather hinge; the case is fitted with fasteners, a short carrying handle, and a long adjustable shoulder strap. This is not a decorative case but genuine field protection: in the Paulin catalogue it was specifically described as a leather carrying case designed for rugged field service, while a separate advertising page emphasised its ability to protect the altimeter under demanding field conditions.

Case, Bezel, and Setting Knob

The instrument itself takes the form of a shallow cylindrical drum. The case is made of metal with a bright white electroplated finish; from its construction and surface appearance it can be identified as a nickel-plated brass case. The front is surrounded by a narrow polished bezel retaining a relatively thick mineral glass with a pronounced bevelled edge. An unusual feature is that the glass has a central opening, through which passes a large circular metal knob with fine knurling. This knob is the instrument’s principal control — the setting knob, connected to a precision micrometer screw. By turning it, the observer does not merely reposition the pointer, but alters the force opposing atmospheric pressure acting upon the sensitive system.

The Dial and Its Scales

The dial consists of a thin brass plate with a matt silvered finish. This surface produces virtually no glare, while all scales, numerals, and inscriptions are executed with exceptional fineness and clarity. The altitude scale is arranged as two concentric tracks, allowing a very wide range to be accommodated around the circumference without excessively crowding the graduations. The inner scale covers approximately −900 to +4000 feet, while the outer scale continues from 4000 to +9700 feet; the major thousand-foot markings on the outer scale are numbered up to 9000 ft, followed by the final hundreds of feet in the range. This arrangement substantially increased the effective length of the scale and thereby facilitated more precise reading.

On the left side of the scale is the characteristic inscription “0 FEET — 0 — 30” MERC.”, linking the zero-altitude mark with a barometric pressure of approximately 30 inches of mercury. At the top of the dial is one of the most important elements of the Paulin System — a small balance window marked “+” and “−”, with a central zero line. Behind it is a reflective surface allowing the position of the extremely fine indicator to be observed without parallax. The central knurled knob on this particular example bears the additional marking G-106.

The Zero-Gauging Method

The fundamental difference between the Paulin and a conventional aneroid becomes apparent precisely when a reading is taken. In a traditional aneroid barometer, a change in atmospheric pressure deforms an evacuated corrugated capsule. Its extremely small linear movement must then be magnified many times by a system of levers, connecting rods, a toothed sector and pinion, or alternatively by a fusee chain, drum, and hairspring return spring. As a result, the final position of the pointer depends not only on pressure, but to some extent also on friction at the bearings, backlash, the elasticity of the transmission components, capsule hysteresis, and the time required for the mechanism to settle. It was precisely these shortcomings of the conventional aneroid that led Paulin to invent a barometer designed to eliminate them.

Paulin adopted a completely different approach. His instrument works according to the zero-gauging method. The sensitive element is used primarily as an indicator of departure from equilibrium rather than as the source of the mechanical movement of the main pointer. Atmospheric pressure tends to compress the evacuated capsules, while an adjustable spring system acts in the opposite direction. For each measurement, the operator manually alters the force of this system until the capsules return to the same precisely defined normal position. The working movement of the sensitive element is thereby limited to approximately 0.001 inch.

When the pressure changes, the capsules move only very slightly from their normal position. This movement is immediately detected by the small tendency pointer in the upper window. When pressure rises, it moves in one direction, towards the + sign; when pressure falls, it moves in the opposite direction, towards −. The observer then begins to turn the central knob. The micrometer screw connected to it gradually alters the tension of the balancing spring system until its force exactly equals the effect of atmospheric pressure upon the aneroid capsules. At that moment the capsules return to their predetermined geometrical position, and the tendency pointer aligns with the central zero mark.

Only then — and not before — is the reading taken. At the same time as it operates the micrometer screw, the central knob mechanically rotates the instrument’s indicating hand — the main pointer — and its position on the altitude scale indicates the elevation corresponding to the equilibrium condition that has been established. Thus, unlike in a traditional barometer, the indicating hand is not driven directly by the aneroid capsules at all — there is no conventional pointer shaft transmitting their motion to the hand. A pressure change may occur and the tendency pointer will immediately indicate its direction, but the main pointer remains at its previous position until the operator carries out a new balancing operation. This has another useful consequence: before each new measurement, the previous reading remains preserved on the scale and can be checked and recorded.

Inside the Movement

The internal mechanism of this example is particularly interesting because it represents the compact version of the Paulin System. The sensitive unit consists of two connected evacuated corrugated aneroid capsules. This twin-capsule arrangement was already present in Paulin’s original patent: two diaphragm boxes were joined into a single sensitive element and subjected to external atmospheric pressure. Surrounding the sensitive unit is an elaborate elastic system. The photographs clearly show a massive curved flat spring element, resembling the C-spring of a conventional movement, together with two symmetrically positioned cylindrical helical springs wound from wire of circular cross-section; judging from the manner in which they are loaded, these are compression springs. Together with the micrometer mechanism, they form the opposing-force system: external pressure tends to bring the walls of the evacuated capsules closer together, while the springs tend to restore the sensitive unit to its predetermined normal geometry. Turning the central knob through the micrometer screw changes this restoring force in extremely small and reproducible increments. Consequently, what the instrument actually measures is not the free movement of the capsules, but the magnitude of the adjustable force required to return them to zero.

The most elegant part of the construction lies in the operating system of the tendency pointer. A lightweight cradle is connected to the sensitive element. At the slightest departure of the capsules from equilibrium, the cradle moves with them. Attached to its two ends are extremely thin flexible strips of phosphor bronze. Their outer ends are fixed, while from their intermediate points two further strips of the same material extend to short arms on the indicator shaft. The result is a symmetrical system of four phosphor-bronze strips. The choice of phosphor bronze is deliberate: these thin strips can flex elastically again and again while producing virtually no backlash and requiring no conventional articulated joints.

The geometry of these strips transforms the extremely small linear movement of the cradle into a much more noticeable rotation of the indicator shaft to which the tendency pointer is attached. The shaft itself, however, is not mounted conventionally in pivot bearings. It is suspended between two resilient spiral elements and therefore rotates about its mathematical axis without traditional pivots or plain bearings. Paulin’s technical description specifically emphasises that this arrangement eliminates the need for a bearing and permits motion to be transmitted to the indicator without bearing friction.

At one end of this shaft is, as already mentioned, the tendency pointer, whose fine tip can be seen in the upper dial window between + and −. It is in fact a rather long pointer, beginning in the lowest part of the movement and extending all the way to the top, curving around the micrometer screw on its way. On the opposite arm is a small counterweight immersed in an oil dashpot — the green glass vessel visible in the photographs. Its purpose is not to create resistance to the measuring motion in the manner of a conventional bearing, but to damp short-term oscillations of the sensitive system: without such damping, the extremely light indicator could tremble in response to the slightest shock, vibration, or sudden movement of the instrument. The operator therefore sees a stable and clearly discernible approach of the tendency pointer towards the zero mark.

The entire kinematics of the Paulin can therefore be divided into two almost independent systems. The first is the force and indicating system: central knob → micrometer screw → change in the tension of the balancing spring → simultaneous mechanical movement of the main pointer across the altitude scale. The second is the sensitive zero system: atmospheric pressure → minute deformation of the two evacuated capsules → cradle → four flexible phosphor-bronze strips → pivotless shaft → tendency pointer. These two systems meet only at the moment equilibrium is established: when the tendency pointer indicates zero, the position of the micrometer screw — and consequently of the main pointer — becomes a measure of atmospheric pressure and of the corresponding altitude.

This is precisely what fundamentally distinguishes the Paulin from a conventional aneroid. Between the capsules and the indicating hand there is no usual mechanical train consisting of successive levers and connecting rods, toothed sector and pinion, fusee or other fine chain, pivoted shafts, and a return hairspring. At the same time, it is more accurate to speak not of the complete absence of friction throughout the instrument — the threads of the micrometer screw and the oil dashpot physically remain — but of the elimination of bearing and transmission friction from the sensitive measuring train, that is, precisely the kind of friction that directly affects pointer position in an ordinary aneroid.

Another consequence of the zero method is equally important. In a conventional aneroid, the capsules operate continuously at different degrees of deformation under different pressures, so their own elasticity, hysteresis, and slow recovery of shape enter directly into the measured quantity. In the Paulin, at every final reading the sensitive element is returned to virtually the same original geometry. This greatly reduces the influence of elastic hysteresis and the so-called lag — the delay in indication following a rapid change in altitude. The company regarded this as one of the principal advantages of the system: the instrument could be moved through changes in elevation and a new reading taken almost immediately, without waiting for a conventional aneroid to “settle” and regain equilibrium.

Conclusion

The Paulin Precision Altimeter therefore occupies a very special place among the mechanical barometric instruments of the first half of the twentieth century. Externally it retains the familiar appearance of a large field aneroid — a circular dial, a single long indicating hand, and a leather carrying case — yet internally its measuring principle is almost the opposite of that of a conventional aneroid. Pressure does not directly force the pointer to move across the scale; instead, it merely disturbs an extremely sensitive mechanical equilibrium, which the operator then restores by means of the micrometer screw. The position of that screw after the zero condition has been restored is what constitutes the measurement.

This solution made it possible to combine the portability of an aneroid with a degree of precision that had previously been much more difficult to achieve in a compact field instrument. What we have here is not merely one of the many altimeters of the 1920s, but a characteristic example of Paulin’s distinctive school of precision instrument making — a system in which the elimination of the traditional transmission mechanism was not simply a means of simplifying the construction, but a way of radically changing the very method of measurement.