A hair hygrometer
This device will enable you to read the relative humidity directly without the use of tables.
Procure a few human hairs about 30 cm long. Free them from grease with dilute caustic soda solution. Fix one to the upper end of a stand and stretch it with a 50 gm weight. The hair should pass two or three times round a spool fixed to an axle which is free to turn in bearings made from a piece of tin and fastened two-thirds of the way down the stand. Fix a Light pointer of balsa wood to the axle, and organize a postcard to act as a scale. For greatest sensitivity the diameter of the spool should be small