» Fortin Barometer (0595)

Immagini

DESCRIPTION

Exhibit 595 is a Fortin mercury barometer for measuring the atmospheric pressure based on the same principle of the Torricelli barometer with respect to which the Fortin version is more precise but requires more effort.

As in the Torricelli barometer, a barometric tube filled with mercury and closed on the top is inserted in a mercury bath where the atmospheric pressure is balanced by the hydrostatic pressure of the mercury column so that:

Where:
ρ is the density of mercury;
g is the acceleration of gravity;
h is the height of the mercury column above the mercury level in the bath.

A disadvantage of the Torricelli barometer is that the level of the mercury in the bath tub varies with the level in the closed tube so that the lower reference point for measuring the mercury column height is not fixed.

To eliminate this inconvenience Fortin designed his barometer with the mercury in a leather sack that rests on a screw that can be turned to reestablish the lower reference point for determining the mercury column height, indicated by an ivory pointer that must coincide with its reflection on the mercury surface.

The vertical tube comunicates with the bath through a hole in its cover and is protected by a metal shield on which the zero graduation corresponds to the ivory pointer. In our barometer the column height varies from 52 to 82 mm and can be read to a precision of 0,05 mm using the sliding vernier provided. The metallic protection of the vertical tube is rectangular in front and behind where a sliding mirror permits reading the top surface without parallax. This strument is suspended from a hook to remain vertical independent of the orientation of the wood backing.

For a precise determination of the atmospheric pressure, it is necessary to consider other factors:

It is necessary to control the ambient temperature that can influence the density of mercury and of the graduated metal shield, for which there is a holder for a thermometer (actually missing).
It is necessary to consider capillarity of the tube if its diameter is less than 2 mm.
It is necessary to correct for the residual gas above the mercury column, usually the saturated mercury vapor pressure.

ISCRIZIONI

Sul contenitore mecurio è inciso:"Gino Ceccarelli Roma; N 102"

BIBLIOGRAPHY

[1] Privat-Deschanel, "Trattato Elementare di Fisica", Milano,1882, §183, pagina 156.

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