Measuring humidity is a large part of being able to forecast weather. Being able to forecast the weather consequently allows individuals to plan their days---a boy scout troop, for example, would want to pack rain gear if humidity was high, or a farmer may turn on the irrigation system to crops when humidity is very low. Although there are many modern tools that measure humidity, one tool that is hundreds of years old and still works for humidity measurement is the hair hygrometer.
Definition
A hair hygrometer is a simple device that is used to measure humidity. Its primary component is human hair, which is hooked up to a lever or gauge system that can provide a means to read percentages (or at least show variation from low to high) of humidity in the atmosphere. The gauge can be something as simple as a piece of cardboard with degree markings. The devices sometimes are hooked up to a roll of paper and a writing tool (a hygrograph) so that the changes in humidity can be documented as the lever shifts positions.
How It Works
Because of the way hair is constructed, it expands as the humidity levels get higher. It contracts when the humidity levels lower. When it is humid, the hair in a hair hygrometer thus gets longer, and the lever connected to it moves in response to the extra slack. When the humidity gets low, the hair in the hygrometer gets shorter and pulls on the lever.
Pros and Cons
Hair hygrometers do not require electricity. However, they have very large errors when relative humidity levels are very low or very high. This is because the hair in the hygrometer can only expand and contract to a certain degree. They also need to be adjusted and calibrated often because of this problem.
History
The first hygrometer is credited to Leonardo DaVinci, but the first hair hygrometer wasn't invented until 1783. Horace Benedict de Saussure is thought to have been the inventor. Hair hygrometers have been used ever since, and were not replaced with electrical humidity measuring devices until the 1960s. This is partly because the use of electricity did not become commonplace until the 20th century, and because technology was not advanced enough to provide more accurate means of humidity measurement.
Modern Use
Because hair hygrometers are very simple to make, and because they do provide a rough means of humidity measurement, they still are constructed despite technology that allows for more accurate readings. They often are constructed in schools as part of weather courses so that students can have a hands-on project related to humidity.
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