A miniature device has been developed, which measures the mean electrical mobility of an aerosol using an electric field to precipitate the aerosol onto parallel plates, between which it is drawn. The electric field is provided by an electret, and so no powered high voltage source is required. Measurements on a prototype indicated that the electret must be long and wide, and separated from a parallel earthed plate by only a small distance, if the deposition pattern is to be easily observed. A large dynamic range is obtained if the charge on the electret is made spatially variable so that the aerosol encounters first a weak and then a strong electric field. The theory of action of the device is discussed, along with experimental results obtained.