An automatic dew-point hygrometer making use of .BETA.-ray backscattering and controlled at the constant amount of dew.

A control system which consists of proportional and integral control to maintain the constant amount of dew was developed in this hygrometer. The dew points were measured within an accuracy of±1 °C in the ranging from -4 to 32°C. The response time for suddenly changing humidity was about 8 min.