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Object Overview
This Balloon Statoscope was made by the celebrated Parisian instrument-maker Jules Richard, circa 1890. The statoscope, or ultra-sensitive air barometer, renders visible exceedingly small changes in atmospheric pressure. Its purpose is not to measure absolute pressure but to register the very fact and direction of its change. The instrument discriminates minute differences: a change of just 1 mm of mercury drives the needle roughly 25 mm, and a change in altitude of less than one metre already produces a clear deflection.
The instrument was devised to give the aeronaut a precise means of telling whether a balloon is rising or falling. It suffices, from the basket, to pinch off the rubber tube and watch the needle: the moment a pressure difference arises between the air sealed inside and the outer atmosphere, the needle at once swings one way or the other. Hence the name — "statoscope."
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rack and pinion
brass, cardboard, glass, leather, paper, rubber, steel, wood
The instrument is housed in a protective case insulated with felt and wool, so as to minimise the influence of temperature as far as possible. Observations with the statoscope are usually brief, so the air within the reservoir has no time to warm appreciably, and no separate temperature-compensation mechanism is required.
The case is made of a light cardboard composite — essentially papier-mâché: multiple layers of sized card with a textile reinforcement, what in the late nineteenth century was called carton-cuir or pressed millboard — faced with cloth, primed and painted. The case weighs only about 500 grams, which is of the first importance for an instrument to be carried aloft in a balloon.
Around the case runs a leather strap with buckles, for carrying the instrument and suspending it in the balloon's basket; for this, plant bast fibre — flax or hemp — was additionally used, its ends passed repeatedly through the eyelets of the strap.
The top bears an insulated lid with a viewing window that reveals the principal portion of the dial with its indicating needle of blued steel. The lid carries the inscriptions:
Around the circumference: TENIR L'APPAREIL SUSPENDU ("keep the instrument suspended") and below it NE PAS SOUFFLER DANS LE TUBE NI LE LAISSER FERMÉ ("do not blow into the tube, nor leave it closed").
These inscriptions make plain at once that this is no ordinary aneroid but a differential air barometer, working close to a single point of equilibrium: it shows whether pressure is beginning to rise, beginning to fall, and how fast this is happening. The instruction to keep the instrument suspended is no formality: the suspension acts as a mechanical filter. Set on a table, so sensitive a mechanism would answer with false readings to every footstep, jolt or gust of wind; its whole adjustment is calculated for use in one orientation.
Why a statoscope is needed
It is vital for the aeronaut to know at any moment whether the balloon is rising or descending, and at what vertical speed. The oldest method is to throw light objects overboard: scraps of tissue paper, ribbons and the like. A sheet of tissue paper falls through still air at about 50 cm/s, and from its position relative to the basket one may judge the balloon's movement. If the basket keeps level with the sheet, the balloon is descending at roughly the same rate, about 50 cm/s; if the sheet lags below, the balloon is descending more slowly, is in equilibrium, or is rising (when the gap opens faster than 50 cm/s); and if the sheets circle above the basket, the balloon is descending markedly faster than 50 cm/s.
The method serves only in still air. When the atmosphere is disturbed, eddies now slow, now hasten the paper's fall, and the readings become plainly false — which is especially dangerous in the mountains, where vertical currents are frequent. The self-recording barograph-altimeter is more reliable, giving a continuous curve of pressure. But at the usual scale of the record (about 1 mm to 1 m of altitude), the direction of movement can be told with confidence only after several tens of metres have been traversed.
It was precisely to remove these shortcomings, fraught as they may be with serious consequences, that the statoscope was created — an instrument that shows automatically, by the deflection of its needle, the direction and relative magnitude of the balloon's vertical movement.
Construction and principle of operation
The statoscope consists of an airtight reservoir of small volume — the metal case of the instrument itself serves as this — communicating with the outside air through a rubber tube. While the tube is open, the pressure on either side equalises and the needle rests at zero. Pinch the tube shut — with a plug, a clip, or simply the fingers — and a volume of air is sealed inside at the pressure prevailing at that instant; this is the reference.
The sensing element is a stack of metal capsules with corrugated walls, communicating with one another through a central tube — in effect a bellows stack, set at the boundary between two bodies of air: the air sealed within the reservoir (the reference) and the outer atmospheric air. Unlike the aneroid boxes of a barometer, there is no vacuum within the capsules — they are filled with air. Boxes of this kind are termed manometric — their internal cavity is connected to the pressure being measured. The slightest pressure difference across their walls flexes the stack; its travel is taken off by a thin rubber diaphragm bearing a light aluminium disc, through which the transmission mechanism reads the movement.
The moment the tube is pinched shut and the instrument begins to rise or fall, the balance of forces on the elastic wall is upset, and it deforms until its elasticity once more offsets the pressure difference that has arisen. This deformation, greatly amplified by the mechanism, is displayed by the travel of the needle. The deflection becomes distinct at a difference in level of less than one metre.
Dial and scale
The dial is of stout card, protected by a wooden lid with its own viewing window, into which a flat sheet of mineral glass is set; this window is aligned with that in the lid of the case. The scale is conventional (empirical), with a few indicative marks:
There are no familiar millimetres of mercury here. The scale reads Descente (descent) and Montée (ascent) — that is, the needle indicates the direction of the pressure change, and hence of the balloon's movement. Above the upper part are the figures 5, 10 and 15 to either side of zero — the relative magnitude of the change.
Transmission mechanism
The mechanism is mounted on a longitudinal brass plate. The receiving link is a taut rubber diaphragm with a light aluminium disc laid upon it; in the working position it is held, by a forked connection on a conical pin, by a flat elastic plate (a leaf-spring holder) fixed at one end to the case. The solution is characteristic of the Richard school: there is no rigid connection between the capsule stack and the mechanism — the whole transmission passes through contact with this plate, which frees the mechanism from lateral displacements and tilting of the capsule and passes on only the vertical component of its travel.
The principal transmission link is a rocking arbor of square section, mounted beneath the plate on pivots in bearing holes; in construction it is an arbor with arms, working as a two-armed lever. From one end of the arbor extends a short receiving arm bent into a hook: it is brought beneath the flat plate and rests against it from below, forming a free, unattached pair. From the opposite side extends a long rod with a weight — a counterpoise — that slides along it. Upward from the arbor, through an opening in the plate, runs a pusher that imparts motion to a toothed sector (a curved rack); the sector engages the pinion of the needle arbor, which carries, besides the needle, a spiral hairspring.
The sequence of transmission is thus: the travel of the capsules under the pressure difference is taken up by the taut diaphragm with its aluminium disc; its movement raises or lowers the hook, turning the arbor on its pivots; the turning of the arbor, through the pusher, shifts the toothed sector, and this rotates the pinion and with it the needle arbor and needle. The small travel of the capsules is turned into a sweeping movement of the needle by the ratio of the lever arms and of the sector-and-pinion pair.
The counterpoise provides the force closure of the whole kinematic chain. Since the hook merely rests on the plate, and the sector is merely pressed against the pinion, the chain is kept in constant contact not by joints but by a moment: the weight on the long arm continually presses the hook upward from below, and that same effort, transmitted along the chain, takes up the clearances, working in concert with the hairspring, which removes backlash in the toothed engagement. Thanks to this the needle follows the diaphragm in both directions without play. By moving the little weight along the rod the instrument is adjusted; at the same time the moving system itself is thus balanced about the axis of the arbor, which matters for a portable instrument.
Zero corrector
On the top lid, to the right of the viewing window, is a knurled head — the zero corrector, the means of setting the needle to zero by hand. Outwardly it is a low milled thumb-wheel on a long brass stem, but the arrangement is more intricate than it seems. The stem with its head terminates in a spring pack: two parallel helical springs clamped between an upper and a lower disc. From the lower disc issues a short threaded shank, screwing into a hole in a brass platform and, in the end, acting upon a slightly bent steel plate for setting zero — by raising the whole mounting plate. The head and the working screw are thus not a single part but two links joined through an elastic pack, a kind of flexible coupling.
The device serves several functions at once. The springs, clamped under preload, continually take up the play of the thread, so that the needle follows the turning of the head with no lost motion; the same effort creates friction in the thread that keeps the screw from turning of itself under vibration — of consequence for an instrument working in flight. Along its axis the pack serves as a shock absorber, cushioning any accidental push on the projecting head. Finally, the lateral compliance of the pack lets the corrector, when the head is knocked sideways, yield in that direction without turning in the thread — so that an accidental touch does not disturb the zero once set. Moreover, two parallel springs transmit rotation more reliably than a single one, while remaining compliant to lateral displacement. The corrector thus separates the deliberate turn from the accidental touch: the first it passes to the mechanism, the second it absorbs.
Conclusion
The Richard statoscope is a model of how a narrow measuring task gives rise to a thoroughly considered design. By forgoing the measurement of absolute pressure in favour of differential, the instrument gains a whole order of magnitude in sensitivity: the capsules need not spring against the entire atmosphere, and so respond to hundredths of a millimetre of mercury — that is, to a mere few tens of centimetres of altitude. Everything else is subordinated to not squandering that sensitivity: the thermal insulation of the sealed air, the decoupling of the mechanism from the capsule's tilt through the flat plate, the force closure of the chain by counterpoise and hairspring in place of rigid joints, the flexible coupling of the zero corrector that damps chance touches. Each of these solutions looks modest on its own, yet together they yield an instrument that reliably discerns the balloon's movement long before a barograph — or a scrap of paper thrown overboard — would notice it. Herein lies the hand of the Parisian school of precision instrument-making of the late nineteenth century, to which Jules Richard himself belongs: sparing in its materials, faultless in conception, easy to repair, and more exact than the eye has any right to expect.