A Hygrometer for Measuring Relative Humidity
Materials such as gut string, wool thread, wrapping twine, etc. absorb moisture from the air and change their lengths. Since these materials are twisted, the change in length is better observed through their change in twist.
Make a strand of wood which is moveable up and down for adjustment to different lengths of the substances we wish to test. Cut a circular disk of 3" radius out of cardboard and make scale divisions along the rim for every 5 degrees, numbering every 10 degrees, with numbers from "0" to "35". The disk is fastened with a thumbtack through the center onto the base board in a plumb line under the hole in the suspension beam and wire loop.
The details of the a pointer, made out of a clothes pin, a piece of stiff wire, about 3 to 4" long, and heavy (nails around 1 oz) is shown in the pointer detail diagram below. The string to be tested is clamped into the pin at the top and attached to the pointer.
Obtain a 10" piece of broken gut string from a violin, viola, or guitar, making sure that the string is not made of nylon. Cut the kinked or curled portion off the string and use only the straight portion. Clamp the string in the plug at the top and the pin at the bottom; the pin should hang horizontal and the pointer should be about 1/4" above the scale.
Let the instrument stand for a few hours, so that the string is stretched to its equilibrium length. Take a reading of the scale and a relative humidity reading with a psychrometer.
Use a wet box to determine the point of 95% relative humidity, and make a calibration chart for this instrument.
Repeat this for several different materials such as wrapping cord and wool thread. Some of the materials have rather large responses to changes in humidity, so that the pointer may turn more than a full circle; in this case, shorten the string.
Material: Adjustable support frame
One circular cardboard disk 6" diameter
One clothes pin
Four heavy 3" nails
One gut string of a music instrument, wool thread and wrapping cord, 10" each
One big cardboard box to fit over instrument and a pan of hot water.