Assmann Hygrometer manufactured by CF Casella & Co Ltd in a wooden box with carrying handle. Instruction Leaflet.
The Instruction Leaflet has a passage under the heading Principle of Operation:
“Two thermometers are arranged side by side in the same plane, one of the thermometers being fitted with a closely woven, well fitting sleeve which is moistened with distilled water. A fan turned by electric or spring driven means provides air movement or aspiration past the two thermometer bulbs. Evaporation from the bulb with the sleeve surrounding it (the ‘wet’ bulb) proceeds until equilibrium with the air stream is reached. This evaporation causes a drop in the wet bulb thermometer reading (the ‘depression’) which, when compared to the dry bulb thermometer reading is a measure of the relative humidity. After a large number of readings taken by various people in the past, formulae have been evolved relating the depression of the wet bulb to the dry bulb reading from which figures have been computed, giving the relative humidity for a large number of temperature readings so that the useful range of the instrument is now covered by suitable tables.”