Wet and dry bulb hygrometer (English spelling)

Japanese: 乾湿球湿度計 - かんしつきゅうしつどけい（英語表記）psychrometer

An instrument that measures the humidity of the atmosphere by the rate at which water evaporates. It is also called a psychrometric meter. Two identical glass thermometers are attached vertically to a board, one with its bulb wrapped in a damp cloth (the bottom end of the cloth is immersed in a water bottle) is called a wet bulb, and the other with its bulb not wet is called a dry bulb. The lower the humidity, the more water evaporates, and the lower the temperature of the wet bulb because the heat of evaporation is lost. The difference in temperature between the wet bulb and the dry bulb, which indicates the air temperature, is measured, and the relative humidity is calculated using a calculation table. The temperature difference is affected by air pressure, radiant heat, water temperature, the size of the bulb, how the cloth is wrapped, and especially the wind speed that hits the wet bulb, so a simplified model that simply attaches both thermometers to a board may have an error of about 5% in the humidity. The August type wet and dry bulb hygrometer, which uses a glass rod thermometer, and the Huth type wet and dry bulb hygrometer, which uses a glass double tube thermometer, have been in use since the mid-Meiji period. In the ventilated type, a constant speed wind is blown on both bulbs to improve accuracy, and the Assmann ventilated psychrometric meter is a typical example.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

水の蒸発の遅速から大気の湿度を求める計器。乾湿計ともいう。同形の 2本のガラス製温度計を板に垂直に取り付け，一方の球部を湿った布（布の下端が水瓶に浸されている）で包んだものを湿球，球部を湿らせないもう一方のものを乾球という。湿度が小さいほど水の蒸発が盛んで，蒸発熱を奪われるために湿球の温度が下がる。気温を示す乾球の温度との差を測定し，算出表により相対湿度を求める。温度差は気圧，放射熱，水温，球部の大きさ，布の包み方，特に湿球にあたる風速に影響されるため，両温度計を板に取り付けただけの簡易型では湿度に 5%程度の誤差を生じることがある。ガラス製棒状温度計を用いたオーガスト式乾湿球湿度計，ガラス製二重管温度計を用いたフース式乾湿球湿度計は，明治時代中頃から使用されている。精度を上げるため，両球に一定の速度の風をあてるようにしたものが通風型で，代表的なものにアスマン通風乾湿計がある。

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