The hygrometer used in the Han dynasty was fashioned after a weighing scale with a feather and a piece of charcoal on either side. If the humidity is high, the scale will tilt to one side as the charcoal becomes heavier after absorbing moisture from the air.
與指南車同樣造形奇特的是濕度計，它形狀像天平，但兩端分別懸上羽毛和木碳，這是西漢時代的濕度計，它兩邊的重量相等，如果天候潮濕，木碳吸了空氣中的濕氣，就會變得比羽毛重而往下垂，可測出空氣中的水份增加，反之則減少。