The hygrometer described is continuous-indicating; a value of the humidity is obtained by balancing a column of atmospheric air against a column of air saturated with water vapor and observing the effect, on a suspended vane, of the difference of pressure due to the different densities. The instrument used in the investigation is more sensitive than the Regnault-Alluard type of dew-point hygrometer and can be made a hundred times as sensitive if required.
ARNOLD ROMBERG and L. W. BLAU, "A NEW HYGROMETER," J. Opt. Soc. Am. 13, 717-721 (1926)