> tape sensor

Indicating mechanical hygrometers are usually made with a flat spiral
spring coated on one side with some sort of hygroscopic spongy substance.
Humidity causes it to absorb H2O and expand a tiny amount. The long spiral
turns this into about 180 degrees of needle movement without using a gear.

All cellulose and similar based materials will change size when wetted.
This includes wood, T shirts, cotton thread, and probably cellulose
acetate which may be the material that tape was made of. Other candidates
are silk and human as well as horse hair.

Peter

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