SM0486
chloride, as the humidity increases. A color comparison scale, when employed with
this indicator, gives a rough estimate of the relative humidity.
Various other
similar chemicals have been used, more or less successfully.
(b) A great many experiments have been performed with various types of
materials for use in electrical hygrometers. Electrical hygrometry is based upon a
change in electric resistance of a hygroscopic material with changes in humidity.
The material may be metal wires with various chemical compounds placed between the
wires, such as polyvinyl acetate, polyvinyl alcohol and lithium chloride,
phosphoric and sulfuric acid, and others.
Plastics, underfired clays, cotton
impregnated with various solutions, and cotton wool and human hair have been
investigated. In some of these materials, the resistance appears to vary directly
with changes in humidity.
In others, resistance appears to vary logarithmically
with changes in humidity.
(c) For specialized purposes, electrical hygrometers have proven to be
superior to other measuring devices, but for weather forecasting and general
humidity control the other types of hygrometers are better suited.
However, it
would seem that, eventually, electrical hygrometers will replace most of the other
types of humidity measuring devices.
(5) This completes our discussion of humidity measuring instruments.
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