Silk-Thread Hygrometer with a Magnifying Sag and Pointer (Boy Mechanic, 1913)

"A very small shrinkage of B, such as occurs when the atmosphere is dry, will cause an increased movement of C."

Drive a nail at each end of a board A and stretch silk thread B between them, not quite straight: let it bend a little toward the pointer bracket D.

Cut a round wooden axle about the size of a lead pencil and drive a pin into each end. Cut a piece of tin E V-shaped at each end and bend the ends up as bearings for the pins; fix it on bracket D. Fix a light pointer on the axle.

Tie a second silk thread C to the middle of B, fasten it to the axle and wrap it once or twice round, so pulling C turns the pointer over a scale card.

When the air dries, B shrinks and straightens, pulling C and swinging the pointer; damp air lets it back. Mark the scale against a known hygrometer if you want numbers.

From **The Boy Mechanic, Book 1** (Popular Mechanics, 1913), pp. 71-72 (checked on the archive.org scan boymechanic0000unse_n5a8). He gives no sizes or calibration; ours are marked.