A hygrometer UK /haɪˈɡrɒmɪtər/ is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation (the dew point), or changes in electrical capacitance or resistance to measure humidity differences. The first practical hygrometer was invented by polymath Johann Heinrich Lambert in 1755.
Metal-paper coil type
The metal-paper coil hygrometer is very useful for giving a dial indication of humidity changes. It appears most often in very inexpensive devices, and its accuracy is limited, with variations of 10% or more. In these devices, water vapour is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape. These changes (analogous to those in a bimetallic thermometer) cause an indication on a dial.
Hair tension hygrometers
These devices use a human or animal hair under tension. The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale. The traditional folk art device known as a weather house works on this principle. Whale bone may be used in place of hair.
In 1783, Swiss physicist and geologist, Horace Bénédict de Saussure built the first hair-tension hygrometer, using human hair.
It consists of a human hair eight to ten inches long, b c, Fig. 37, fastened at one extremity to a screw, a, and at the other passing over a pulley, c, being strained tight by a silk thread and weight, d.
—John William Draper, A Textbook on Chemistry
The pulley is connected to an index which moves over a graduated scale . The instrument can be made more sensitive by removing...
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