The hair hygrograph is an instrument used for recording continuously the relative humidity of the air. The sensor of this instrument is a bunch of human hair. The length of the hair increases as the humidity increases and vice versa. These small changes in length are magnified by means of a lever system such that the movement of the pen is proportional to the relative humidity. The hair hygrograph is kept in the double thermometer screen along with the bimetallic thermograph.
