Abstract
Using very fine dry- and wet-bulb thermo-junctions an instrument has been designed which, by simulating electrically the behaviour of the psychrometric equation for small changes in the variables, provides a means of recording the fine structure of atmospheric vapour pressure. In its present form the instrument provides a record of the vapour pressure fluctuations above some arbitrary level, but it could be adapted for use as a probe for recording actual vapour pressures in confined spaces.