A method is described by which the presence of adsorbed moisture on the surface of a solid insulating material in the form of an ellipsoid freely suspended in a closed vessel can be observed and its electrical properties measured.The theory of the instrument is set out, together with a numerical example, and expressions are found for the polarization and conduction currents, and for the resistivity of the film on glazed porcelain. This is of the mean order of 102ohms per cm2. An example is given of the change of the of the film with time.Chemical and physical drying are compared, and a form of electrical hygrometer is described depending on the change of distribution of electric fields on a glazed surface produced by a change in the hygrometric state of the air. By the use of a relay an alarm-bell is rung when a certain hygrometric state is reached.