air into the vacuum chamber; in others, the long lever extending from the steel service spring is made of two strips—brass and steel— brazed together. The altered length of the lever is made to offset the weakening of the service spring. An aneroid of this construction is thereby compensated for temperature. A barometer bearing the name of the maker, and not a fictitious name, is pretty apt to be as it is represented. Among commercial aneroid barometers one may find instruments thus marked in which the compensation is far from perfect.
When the aneroid is to be used wholly to indicate weather conditions, compensation, although desirable, is not essential. On the other hand, if it is to be used mainly for measuring altitudes, compensation must be regulated with extreme care. A compensated aneroid requires no temperature correction; the compensation is for the purpose of eliminating such corrections.
The Goldschmidt type of aneroid differs from the type commonly known by dispensing with the train of levers. A micrometer screw working in the cover of the box measures on its graduated rim the amount of the movement of the corrugated top of the vacuum chamber. When operated by an expert trained to the use of micrometry, this type of aneroid possesses many merits. It is not well adapted to general use.
At the best, the aneroid is a delicate instrument requiring great care, especially if it is a part of an engineer’s equipment. The reading of an instrument having a large dial will change with any material change in its position. If the user watches its variations, however, such erratic changes—and they are small—will not result in erroneous readings.
Adjustment of Aneroid Barometers.—An aneroid barometer is sometimes blamed because it does not agree with the reading of a mercurial barometer at the same level. This may occur when rapid changes in pressure are taking place. An aneroid of the best type is very sensitive. It responds to changes in pressure far more quickly than does a mercurial barometer. Because of its complex mechanism it is easily put out of adjustment; moreover, it will get out of adjustment for causes that are not well known.
Adjustment to a correct reading should be made, if possible, when pressure is stationary. The adjustment is made by means