​This Airguide hygrometer probably dates from the 1950s.  It uses a metal-paper sensor coil directly connected to the indicator needle.  The coil consists of a salt-impregnated paper strip attached to a metal coil.  As the paper strip absorbs or releases moisture it causes the metal coil to wind/unwind, much like a bimetallic thermometer coil.  The use of metal-paper coils is generally limited to inexpensive instruments as the accuracy is roughly +/-10% relative humidity. However, it does provide a qualitative indicator of humidity level for home use.

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