A stream of air whose pollution is to be measured is forced between the plates of a first and second air capacitor having a suitable length and applied voltage for furnishing a first and second measurement signal varying, respectively, as a function of small and large positive ion concentration in the air. A divider circuit divides the second by the first measurement signal. The resultant first output signal is added to a second output signal similarly derived from the measurement of negative ions to furnish the final pollution measuring signal.

http://www.freepatentsonline.com/3889180.html