A new instrument to measure the charge-to-diameter ratio of small particles (2-100μm) is described. In this instrument, the terminal velocity of charged particles in an electric field in air is determined by measuring the time of flight of the particles between two laser beams. The main a of this instrument are that it is easy to operate, and only about 15 min. are required to obtain a distribution of charge-to-diameter ratio for a few thousand particles. The major disadvantage is that low charged and uncharged particles cannot be detected.
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