Barometers Realm

The barometer was sold “for parts” as a non-functional instrument with missing components (glass, indicator hand, feet, and suspension ring). However, the extremely rare movement inside was completely intact, and I was able to fully restore the instrument’s functionality.

The open dial of the barometer is made of cardboard, featuring a concentric barometric scale calibrated in millimeters of mercury. The outer ring displays seven textual weather indications in French. At the bottom, the instrument is labeled “Baromêtre Anéroïde.” The replacement indicator hand is made of blued steel. The dial is protected by a new flat mineral glass, but it lacks the pointer trend mechanism, though the original indicator hand remains present. The glass is held in place by a brass bezel, fitted over the case.

The open dial design allows a view into the interior of the instrument, where the unique rackless movement of the barometer is visible.

Immediately behind the indicator hand, a brass bar is visible, secured with two screws to its supports. This bar holds the entire pointer shaft, which is surrounded by a small helical spring. A linked chain is wound around the pointer shaft and extends downward, attached to a thin steel rod, which we will return to later.

Looking deeper inside, we see a wide horizontal brass plate—the main bracket—resting on supports fixed to the main frame. Beneath this main bracket, between two holders, a delicate brass rocker arm is mounted transversely. The upper part of this rocker arm forms an unusual shape and acts as a counterweight, while the lower straight section extends into a long lever, ending in a pressing arm shaped like an arrow pointing upward.

The tip of this pressing arm is inserted into a round hole in the main lever, but its tail end is directed downward at a right angle to the lower plate, freely resting (without any attachment) on a small angular lever. The plate with the angular lever, like the rocker arm, pivots between two holders.

Completing the closed-loop movement transmission, another lever extends upward from this plate and connects to the previously mentioned steel rod and chain, leading to the pointer shaft.

But how does the initial movement of the aneroid capsule walls reach the rocker arm? Typically, a tiny pusher soldered to the upper sensitive chamber wall is used to transfer this initial weak motion to the rocker arm for further amplification.

In this case, however, a small screw is used, clearly visible through the dial, positioned slightly above the pointer shaft. This screw is inserted into the base of the rocker arm’s counterweight, pressing down onto the aneroid capsule. This provides an additional adjustment feature. It is through this screw that the initial movement of the sensitive chamber walls is transmitted to the pivoting rocker arm, which then acts as a long lever, freely pressing from below on the angular lever of the oscillating plate. The movement is then transferred to the vertical lever, which is connected to the steel rod and chain, ultimately rotating the pointer shaft.

The final element of the construction—the sensitive aneroid capsule by Vidie, 45 millimeters in diameter—features a noticeable elevation at the center, devoid of the usual corrugated structure. Inside the capsule, there is a built-in helical spring, preventing the collapse of the chamber walls under atmospheric pressure.

How is the standard aneroid barometer hand calibration performed in this case? On the back surface of the instrument, there is a calibration hole containing an adjustment screw. However, the method used in this barometer breaks all conventions.

The reason is that the supports for the plate with the angular and vertical levers, where it pivots between its holders, are not positioned on the main frame as usual. Instead, they rest on a long flat steel plate, which is itself attached to the base of the aneroid chamber and slightly extends beyond its diameter on one side.

Through the main frame, a screw is inserted, with its sharp tip pressing against this steel plate. By turning the screw, one can influence the entire plate, raising or lowering it. This, in turn, raises or lowers all the crucial movable lever connections positioned on the edge of this plate.

I won’t judge the effectiveness of such a solution, but raising the plate too much with the screw can quickly cause it to press against the sensitive chamber itself, effectively blocking the entire movement mechanism.