A hygrometer measures humidity or the amount of moisture in the air. This is done by measuring the change in length of an organic fibre (e.g. human hair) brought about by the absorption of moisture. A hair hygrometer may be designed to activate
a pen on a clock -driven drum. Such an instrument, called a hydrograph, gives a continuous record of fluctuations in humidity over a given time period.
The human hair cells will indicate the level of moisture in the air by expanding (when air is moist) and contracting (when air is dry).