Page:Handbook of Meteorology.djvu/218



The aneroid barometer has become an instrument of the greatest usefulness to the explorer, the meteorologist and the airman. To the last named it is indispensable; no other form of barometer to take its place has been devised. Its great virtue is its portability.

The Construction of the Aneroid Barometer.—The essential part of the aneroid barometer is the shallow metal box with thin corrugated top and bottom, usually of German silver, having a thickness of 0.004 inch. The corrugation gives a much greater degree of expansion and contraction than would be possessed by a plane surface.

The box is the vacuum chamber or cell. The top and bottom are so elastic that, when the air is exhausted, they collapse almost completely. To prevent permanent warping a stout steel spring attached to a stud in the top of the vacuum box pulls it into a normal position. This mechanism results in a surface that is very sensitive to atmospheric pressure.

A train of levers, a chain and a drum translate the movements of the vacuum chamber covers, caused by changing pressure, into a circular motion; an index pointer moves back and forth over an arc graduated to represent inches of mercury. A movable scale with a zero point that can be set at a desired position encircles the barometric pressure scale. This scale is graduated to express altitudes.

The steel service spring and the metal of the vacuum chamber are weakened by warmth, thereby impairing the accuracy of the readings. In some aneroid barometers this is offset to a considerable degree by the admission of a small portion of dry