The electric hygrometer consists of an aluminum tube coated with polystyrene and wound with a bifilar winding of bare palladium wire. The unit is then coated with a thin film of polyvinyl acetate with an amount of lithium chloride in it depending upon the humidity range to be covered. The electrical resistance of the film between the two coils is a function of humidity. If appreciable temperature changes are involved, a correction is applied.
The thin aluminium tube enables the unit to quickly assume the temperature of the air as it must if measuring relative humidity. The palladium wire prevents an increase in resistance caused by a film which forms on the surface of other types of wires. The polystyrene forms an excellent water-resistant surface of high electrical resistance for the wire and water sensitive film, which prevents a hysteresis effect caused by the absorption of water by glass and other materials originally used as bases. The polyvinyl acetate forms a porous binder for the lithium chloride which gives stability and uniformity to the units without appreciably decreasing their rapid rate of response.
With several units of different sensitivity in parallel, with resistors in series with each, humidities from 10 to 100% may be measured using a single scale of an indicating instrument.
Electrical hygrometer units have been made which have not varied more than 2 to 3% over a period of several months.
Flight tests with two radiosondes carried by the same balloon, one using the electric hygrometer and the other the hair type hygrometer, showed marked changes in humidity recorded by the electric hygrometer which the hair unit failed to register.
*Publication approved by the Director of the National Bureau of Standards of the U. S. Department of Commerce. A preliminary paper appeared in the Bulletin, for June 1938; part of the present paper has appeared in the Nat. Bur. Stand. Jn. Res., Dec. 1939, pp. 701–714, but new material is added here. —Ed.