The aneroid barometer and altimeter
Barometers of the aneroid (without liquid) type are commonly used as weather glasses, the idea being that low pressure, or a sudden fall in pressure, generally indicates unsettled weather while a rising barometer or high pressure is associated with fine weather. The essential part of an aneroid barometer is a flat cylindrical metal box or capsule, corrugated for strength, and hermetically sealed after having
been partially exhausted of air (Fig. 10.14). Increase in atmospheric pressure causes the box to cave in slightly, while a decrease allows it to expand. The movements of the box are magnified by a system of levers and transmitted to a fine chain wrapped round the spindle of a pointer. The chain is kept taut by means of a hairspring attached to the spindle, while the pointer moves over a suitably calibrated scale. (See also Fig. 10.15.)
Aneroid barometer movements are also used in the construction of altimeters for aircraft. In these the scale is calibrated in metres of ascent. Roughly speaking the pressure falls by 10 mmHg per 120 m of ascent in the lower atmosphere.