From Glossary of Meteorology
An apparatus that counts the number of unit charges of electricity that are contained in a sampled volume of the atmosphere.
The general procedure is to pass a sample of the atmosphere through a charged cylindrical condenser. The change in the potential across the condenser is a measure of the ionic charge contained in the sample volume. The change in potential depends upon such factors as the polarizing potential of the condenser, the mobility and charge of the ions, volume and length of the condenser, and sample flow rate. An ion counter is an ionization chamber in which there is no internal amplification by gas multiplication.