Torsion hygrometer

From Glossary of Meteorology

torsion hygrometer

A hygrometer in which the rotation of the hygrometric element is a function of the humidity.

Such hygrometers are constructed by taking a substance in which length is a function of the humidity and twisting or spiraling it under tension in such a manner that a change in length will cause a further rotation of the element.
See hygroscope.