A hair hygrometer uses a bundle of human hair, or from the hair of certain varieties of goat, to measure the relative humidity. As the amount of water vapor in the air changes, the length of the bundle of hair (1) increases (2) or decreases (3) and this movement is amplified by a series of levers (4) to which the bundle is attached and recorded on a moving roll of paper (5).
A hygrometer is a device used to measure humidity (the amount of water vapor the air holds). Usually, hygrometers measure relative humidity, i.e., the amount of moisture in the atmosphere as a percentage of the saturation level at that temperature and pressure.
The hair hygrometer shown in the diagram, though of limited accuracy, is common. The length of a hair increases with increase in relative humidity. This length change is amplified by a lever and registered by a needle on a dial.
The wet and dry bulb hygrometer (psychrometer) has two thermometers mounted side by side, the bulb of one covered by a damp cloth. Air is moved across the apparatus (e.g., by a fan) and evaporation of water from the cloth draws latent heat from the bulb. Comparison of the two temperatures, and the use of tables, gives the relative humidity.
The dew point hygrometer comprises a polished container cooled until the dew point is reached; this temperature gives a measure of relative humidity.
The electric hygrometer measures changes in the electrical resistance of a hygroscopic (water-absorbing) strip.