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The paths of particles during electrostatic precipitation were photographed using a low-power, long focal length microscope. The illumination was periodically interrupted so that the path of a particle appears as a series of dashes. The velocity of the particle at any portion of its path can be computed from the length of the corresponding dash. The particles, studies ranged from 40 to 200 Â¿m diameter. Contrary to the assumptions that have usually been made in theoretical treatments of precipitation, impact phenomena at the surface appear to be very important, particularly for particles over 100 Â¿m. Large particles may rebound without losing their negative charge or they may knock off agglomerations of previously deposited dust which have a positive charge and so are accelerated away from the collecting electrode. When back ionization occurs the paths of particles become quite irregular. Occasionally particles are observed to reverse their charge in a few milliseconds indicating that there are small regions of quite high density of positive ions.