ELECTRICAL ASPECTS OF EVAPORATION OF ICE

Monitoring of the electrical charging of a polycrystalline ice specimen, due to evaporation in vacuo, showed it to be negligible compared with that found by J. Latham and C. D. Stow, the largest current recorded being - 1·09 fA cm-2. Placing the specimen in a negative field did not increase the charging, but moved the readings in a positive direction. When the ice specimen was evaporated in vacuo, a temperature gradient was found to exist. At a pressure of 0·5 torr and evaporation rate of 4·53 × 10-5 g s-1 this was found to be approximately 3 degc cm-1. The evaporation rate was found to increase rapidly with an increase in ambient temperature, readings being taken in the region - 5 to 20°C. Scanning electron micrographs of evaporating polycrystalline ice are presented. These show that the ice developed fine surface structures less than 1 μm in thickness. An explanation of the results of Latham and Stow could be that these very small fragments of the ice specimen were broken off and carried away by the nitrogen stream.