hygrometer using a hygroscopic substance (for example, diphosphorus pentaoxide, P₂O₅) which is transformed into an electrolyte (phosphoric acid, H₃PO₄) in contact with the moisture in the gas. The electrolyte (phosphoric acid) is electrolysed continuously and the electrolysis current is measured. At a constant flow of the gas to be analysed, the electrolysis current is a linear function of the water concentration.