Hygrometers are weather forecasting devices that measure humidity levels in the air. Changes in humidity, in turn, indicate the likelihood of pending dry weather or precipitation. Hair hygrometers like the one shown below, used horse hairs to measure and (sometimes) record air humidity.
This clever design works because hair relaxes and lengthens when humidity in the air increases, and contracts when the humidity decreases. Because of this predictable response to humidity changes, hair can be used to alternately pull and release a lever in response to humid and dry conditions. This device essentially records changes in the length of horse hairs as a proxy for changes in humidity.
The movements are then tracked by a nib which traces an ink along paper on a rotating drum. The drum is rotated once a week by an eight-day Clockwork. The hygrometer shown here is over 50 years old and continues to track ongoing changes in humidity on a week -long chart.
This hygrometer was made sometime after 1890. We know this because the company name shown in the image was only used after 1885 when Henry J Green took the company over from his Uncle, instrument maker James Green. HJ Green then moved from New York to Brooklyn in 1890. Although the measurements are not as precise as could be achieved with other contemporary weather prediction devices, and the accuracy is particularly limited in conditions of extremely high or low humidity, hygrometers reduced the need for mercury based technologies, and provided sufficient information for their typical uses at the time.
Hair hygrometers can be embellished into the folk-art style ‘weather house’. These devices resemble cuckoo clocks, but the moving figures respond to air humidity. There is usually a man and a woman, each in a door frame in the cottage, as shown in the photo below. This example also features a thermometer placed between the two doors.
The figures stand on a balance bar, which is suspended by hair. Using the same relationship between hair and humidity as the hygrometer shown above, the figures are pulled back and forth such that one figure emerges from the house on arid days, while the other is turned out on humid days. The chimney at the top of the house can be turned to manually rotate the figures before you use the device.
If you set the female figure to be outside the house on a humid day, she will always be rotated out on humid days, and vice versa. Once set, you will have a beautiful and accurate hygrometer that is only slightly less convenient than looking out of your window.
Trowbridge, CC. (1896). The use of the hair hygrometer. Science. 4(81):62-65
*By Simone Philpot & Terry Philpot.