Thermo-hygrograph

These devices measure and record the air temperature and the humidity. The depiction is made in form of two parallel running graphs on a paper strip chart. A bimetallic thermometer is employed for measuring the temperature. A recorder draws the movements of the bimetal on the strip chart. Human hair or synthetic fibres are used to measure the relative air humidity. Increasing humidity causes the lengthening, decreasing humidity the shortening of the hair/fibres. The changes in length are recorded on the strip chart as well.