An electronic method of detecting impurities in the air

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Abstract
Impurity concentrations in clean dry air of one part in 107 by weight of polar vapours or of one part in 104 by weight of non-polar vapours were measured in terms of changes in surface potential of a prepared plate. Changes of potential of the order of 10 μV were measured using a vibrating condenser technique and a selective phase-sensitive amplifier. The results suggest that whilst the adsorption is caused mainly by electrostatic attraction between the dipole and its induced image, a small contribution is also caused by van der Waals attractive forces. The possibilities of obtaining selective adsorption, and some of the practical applications are discussed briefly.