Museo Galileo
Virtual Museum
Psychrometer - wet and dry bulb thermometer
    • Setting:
      Room XIV
    • Maker:
    • Date:
      second half 19th cent.
    • Materials:
      brass, glass
    • Dimensions:
      height 440 mm, width 116 mm
    • Inventory:
    • Psychrometer - wet and dry bulb thermometer (Inv. 3467)

This psychrometer measures atmospheric humidity with two thermometers. The frame is fixed on a round brass base. The two mercury thermometers with Réaumur scales are placed on a glass plate. The bulb of one thermometer (called "wet") is wrapped in gauze connected to a wick soaking in a cylindrical vessel filled with water. This arrangement keeps the "wet" thermometer bulb constantly damp. The velocity of evaporation of the water that wets the wick is inversely proportional to the quantity of water vapor present in the atmosphere: the drier the air, the quicker the evaporation. The wet thermometer is cooled by evaporation, which extracts heat. It therefore indicates a temperature lower than (or equal to) that of the other, "dry" thermometer. Knowing the two temperatures, we can determine the relative humidity of the air by means of a table.