Science Fair Project Encyclopedia
Barographs use one or more aneroid cells acting through a gear or lever train to drive a recording arm that has at its extreme end either a scribe or a pen. A scribe records on smoked foil while a pen records on paper. The recording material is mounted on a cylindrical drum which is rotated slowly by clockwork. Commonly, the drum makes one revolution per day, per week, or per month and the rotation rate can often be selected by the user.
Because the amount of movement that can be generated by a single aneroid is minuscule, up to seven aneroids (so called Vidie-cans) are often stacked "in series" to amplify their motion. The french Lucien Vidie (1805-1866) was the inventor.
As atmospheric pressure responds in a predictable manner to changes in altitude they may be used to record elevation changes during an aircraft flight. Barographs are required by the FAI to record certain tasks and record attempts associated with sailplanes. A continuously varying trace indicates that the sailplane has not landed during a task, while measurements from a calibrated trace may be used to establish the completion of altitude tasks or the setting of records.
Nowadays, mechanical recording barographs have commonly been superseded by electronic weather instruments that use computer methods to record the barometric pressure. These are not only less expensive than mechanical barographs but they may also offer both greater recording length and the ability to perform further data analysis on the captured data including automated use of the data to forecast the weather.
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