A Simple Hygrometer

Abstract

THE method described here has been developed to measure the humidity of the air in very small and inaccessible places. The hygrometer is extremely simple, consisting of a piece of paper 1 cm. × 1.5 cm. in size, weighing about 10 mgm. The paper has a hole at one end, by which it can be hung on to a torsion balance and weighed. Paper is a hygroscopic substance, which takes up an amount of water from the air proportional to the relative humidity, irrespective of temperature. The paper used (thin writing paper) increases in weight by 12 per cent when transferred from perfectly dry air to air which is 90 per cent saturated, and reaches equilibrium in 10–15 minutes. The torsion balance used is accurate to one-fortieth of a milligram, and the humidity can be measured accurately to 2 per cent.