Hair hygrometer by Bate, 1830-1840.
© Science Museum / Science & Society Picture Library
In 1783, Horace Benedict de Saussure (1740-1799), a Swiss physicist and geologist, made the first hair hygrometer, using a human hair to measure humidity. This type of mechanical hygrometer makes use of the principle that organic substances expand and contract according to changes in relative humidity. The device was calibrated by exposing it to extremes of total saturation and dehydration, thereby establishing the hair's length at 0 and 100 % humidity respectively.