Anomalous Reading of Hygrometer

Abstract

ON June II I observed a case of the wet-bulb thermometer reading higher than the dry bulb, which cannot be attributed to a falling temperature, as this anomalous condition continued for more than two hours, during the greater part of which the temperature was slowly rising. A gradually dispersing fog prevailed at the time, and the dry bulb was at first covered with precipitated moisture, but after being wiped dry it continued to read lower than the wet bulb, without any further visible deposition of moisture. This, however, may only imply that evaporation was proceeding too rapidly to allow of the fog particles aggregating into visible drops. This evaporation might account for the temperature of the dry bulb being as low as that of the wet bulb, but not lower. As thermal equilibrium will be attained by each thermometer when its rate of heat-loss is equal to its rate of heat-gain, and as the only loss of heat is by evaporation, which at most can only lower the dry-bulb reading to that of the wet, it is necessary to suppose that the wet bulb absorbs heat more rapidly than the dry. This may be accounted for by the greater thermal diffusivity of the wet bulb with its saturated muslin covering.