Saussure's hygrometer is an absorption hygrometer, created by the naturalist Horace Bénédict de Saussure (1740-1799) in 1775. It is based on the property of a well-degreased (for example with a solution of potassium or sodium carbonate, or sulphuric ether) human hair to absorb atmospheric water vapour in proportion to the amount of humidity in the air and to increase in length. The instrument shown here has a brass frame (width 10 cm, height 30 cm) and a wooden case with panes of glass and small side windows with netting; the perfectly degreased hair is fixed to the frame at the top by means of a small clamp the height of which can be adjusted with a screw; at the bottom it is wound around a pulley to which it is attached. A pointer, joined to the axis of the pulley, amplifies the variations in the length of the hair on a scale with 100 divisions.
In correspondence to division 0 and division 100 we see the words: Secheresse Extreme and Humidité Extreme.
The fixed points 100 and 0 were determined by Horace Bénédict de Saussure by placing the instrument under a bell in which the atmosphere was first saturated with steam and then dried with the introduction of melted potassium.
Since the tension of steam depends on temperature, the measurements performed were reliable only at temperatures close to the one at the time of calibration. For this reason, and because hairs deteriorate with the passing of time, Saussure's hygrometer was soon abandoned.


Bibliografia:
Desains (1857), T. I, p.314
Ganot (1864), p. 274
Privat Deschanel (1869), p.372
Turner (1987), p.255
Verdet (1868), T. I, p.135


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