As marvellous an invention as the aneroid barometer was, it had many shortcomings, most of which were approached by various instrument makers over the 100 years or so of production.
Resolution, perhaps expressed as the reading to the highest number of decimal places, was an illusive goal. The problem relates to the expansion and contraction of the cell over a given pressure range: for example the cell may extend by as little as 0.013” when responding to a rise in pressure from 28” – 31” of mercury. The aneroid barometer is essentially a mechanical amplifier comprising a series of levers. A lever may either amplify displacement or force, one always at the expense of the other - nothing is for free!
So if we want to translate the extension of the capsule, 0.013”, onto a dial, we might have to amplify that original signal 800 times, and here we find ourselves wrestling with how to achieve greater resolution.
There have been several attempts to address this issue, but perhaps the most elegant solution I have seen is found in the Daniels’ Patent No 2457, 12 February 1890, demonstrated on vavasseur-antiques.com in two instruments - one made by the fine maker, JH Steward, the other retailed by, and possibly made by, R&J Beck, the Beck being the earlier instrument.
In both instruments, the bridge is very obviously the same, a likely arrangement resulting from the patent and the licence to manufacture being given by John Thomas Daniels to Steward and Beck. Execution of the engineering is quite different in both, suggesting very strongly that they were created by different makers.
Looking at the Patent, Daniels’ central claim is that: “This invention relates to improvements in aneroid barometers whereby the variations in atmospheric pressure may be rendered more readily apparent and indicated with greater accuracy than can be done by the ordinary single needle barometer, the movements of which usually extend over a fractional proportion of the dial.
My invention consists in employing two needles each travelling around a separate dial, one dial being graduated to indicate the inches or other units of pressure and the other graduated to indicate fractions or subdivisions thereof. To effect this I fix a toothed pinion to the upper end of the axis which usually carries the pointer at the same time connecting the chain and hair spring to this spindle in the usual manner.
Gearing with the pinion aforesaid are two smaller pinions one of which carries on its axis the pointer for indicating the inches or other units of pressure whilst to the axis of the other is fixed the pointer for indicating the subdivisions of the said units. The dials may be adjusted to zero by any suitable means. To diminish friction the edges of the larger pinion may be bevilled [sic] so as to reduce the amount of surface contact between the teeth of this pinion and those of the other two. The relative size of the pinions is arbitrary and may be varied according to circumstances.”
On this Steward example, one rotation of the dial is 2,000ft, with the subsidiary dial (inset) having an outer scale marked in feet with a range from 0 – 9,000ft. This instrument is read from the subsidiary dial, the parts of this scale divided to 100ft, and the fractions of 100ft might then be read from the outer open scale. This means that the resolution is actually to 5ft, or at least that is what the maker and inventor would like to imply.
So to the engineering: in essence this is a standard aneroid barometer of the C spring pattern, having a single pressure sensing cell. To this a small gearbox has been added with a substantial base plate raised on a solid pillar or pillars. The primary gear is maintained in a sub-assembly with micro adjustment for depth and angle. The two gears are in fact about 0.010” thick, minimising friction at the teeth. This arrangement requires extremely precise engineering, near perfect alignment being essential, hence the sub-chassis with micro adjustment.
The gears are the obvious focus of attention. Other areas of unwanted friction resultant from the design principally found in the supporting arbors and pivots, the signal from the pressure sensing cell is transmitted to the primary gear via a fine chain, rotation is then distributed to the coarse scale or subsidiary dial, additionally calibrated in inches of mercury, as well as to the fine scale calibrated in feet, altitude.
Daniels’ Patent is a significant development of the basic Vidi instrument.
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