Museo Galileo
Virtual Museum
Maiocchi's tension hygrometer
    • Setting:
      Room XIV
    • Inventor:
      Giovanni Alessandro Maiocchi
    • Maker:
    • Date:
      ca. 1845
    • Materials:
      brass, glass, iron
    • Dimensions:
      height 280 mm, base diameter 92 mm
    • Inventory:
    • Maiocchi's tension hygrometer (Inv. 1381)
    • Maiocchi's tension hygrometer (Inv. 1381)

"Vapor tension" hygrometer invented by Giovanni Alessandro Maiocchi. The apparatus measures the value of the tension of water vapor needed to reach saturation tension. It consists of a glass tube closed at the bottom by an iron faucet, to which is connected a second, vertical, open tube of smaller diameter. At the top, the larger tube is closed by a pair of faucets fitted with a small funnel. A reference ring slides along the larger tube. The ring carries a millimeter scale and a vernier that slides along the smaller tube. The tubes are filled with mercury. By regulating the faucets, part of the mercury can be made to flow out from the bottom of the hygrometer, sucking into the larger tube the air whose degree of humidity has to be measured. After setting the reference ring and the scale so as to coincide with the mercury level in the tubes, a few drops of water are introduced into the larger tube. This establishes the saturated-vapor pressure, which causes the mercury to rise in the small communicating tube fitted with a scale. This increment is equal to the vapor tension needed to reach saturation tension at that temperature. The relative humidity of the air is thus obtained.