A Hygrometer is an instrument used to measure the humidity of the air. A mechanical Hygrometer uses human or horse hairs which stretch as they absorb moisture. It is an instrument that can range from something that is really basic in design, such as a psychrometer or the hair hygrometer, on up to more detailed instruments, like a cooled mirror dew point hygrometer, which actually measures humidity using the condensation temperature.
Lecture on Hygrometers
