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Electrical MobilityWhen a charged particle in a gas or liquid is acted upon by a uniform electric field, it will be accelerated until it reaches a constant limiting velocity. The "electrical mobility" of the particle is defined as the ratio of this limiting velocity to the magnitude of the field. Electrical mobility is proportional to the net charge of the particle. This was the basis for Millikan's demonstration that electrical charges occur in discrete units, whose magnitude is the charge of the electron. Electrical mobility of spherical particles much larger than the mean free path of the molecules of the medium is inversely proportional to the diameter of the particles; for spherical particles much smaller than the mean free path, the electrical mobility is inversely proportional to the square of the particle diameter. Electrical mobility is the basis for electrostatic precipitation, used to remove particles from exhaust gases on an industrial scale. The particles are given a charge by exposing them to ions from an electrical discharge in the presence of a strong field. The particles acquire an electrical mobility and are driven by the field to a collecting electrode. Instruments exist which select particles with a narrow range of electrical mobility, or particles with electrical mobility larger than a predefined value. The former are generally referred to as "differential mobility analyzers". The selected mobility is often identified with the diameter of a singly-charged spherical particle, thus the "electrical-mobility diameter" becomes a characteristic of the particle.
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