MEASUREMENT OF THE ELECTRICAL MOBILITY OF AEROSOLS USING AN ELECTRET WITH SPATIALLY-VARIABLE CHARGE - THEORY AND EXPERIMENT

The use of an electret to measure the electrical mobility of airborne dust particles has been investigated, and a miniaturised device for th is purpose has been developed. The electret, which forms one of a pair of parallel plates, gives rise to an electric field, which precipitat es the particles with an efficiency that increases with their electric al mobility. Air is drawn between the plates at a prescribed rate, and the length of electret over which the aerosol is deposited can be rel ated to the mean electrical mobility, provided that the spread in elec trical mobilities is small. The range of the device is considerably im proved if the charge level of the electret is made small at the point of entry of the aerosol and large further downstream. The theory of ac tion of the device is elaborated, and experimental results obtained wi th aerosols of known electrical mobility are reported.