How to Make a Hygrometer at Home

A homemade hygrometer, for determining the degree of moisture in the atmosphere, is shown in the accompanying sketch and consists of a board, A, with a nail at each end to hold the silk thread B.
A second piece of silk thread, C, is tied to the center of B and connects with an indicating hand or pointer supported by the bracket D.
The axle on which the pointer revolves consists of a piece of round wood, about the size of a lead-pencil, with a pin driven in each end. A piece of tin, E, is cut V-shaped at each end and bent up at the ends to form bearings for the pins.
The silk thread C is fastened to the wooden axle and is wrapped one or two turns around it, so that when the thread is pulled the pointer will move on the scale.

It will be noticed that the thread B is not perfectly straight, but bends toward D. For this reason a very small shrinkage of B, such as occurs when the atmosphere is dry, will cause an increased movement of C, which will be further increased in the movement of the pointer.
An instrument of this kind is very interesting and costs nothing to make.

Excerpt from the book:
THE BOY MECHANIC
VOLUME I
700 THINGS FOR BOYS TO DO
WITH 800 ILLUSTRATIONS
1913, BY H. H. WINDSOR CHICAGO
POPULAR MECHANICS CO. PUBLISHERS