From Glossary of Meteorology
A class of instruments employed to determine the electric potential at a point in the atmosphere, and ultimately the atmospheric electric field.
All collectors consist of some device for rather rapidly bringing a conductor to the same potential as the air immediately surrounding it, plus some form of electrometer for measuring the difference in potential between the equilibrated collector and the earth itself. Collectors differ widely in their speed of response to atmospheric potential changes. When a flame is used to increase the local ion density and thereby facilitate potential equilibration, some advantage over a simple exposed conductor is gained; the relaxation time becomes of the order of one minute. Use of radioactive coatings, preferably emitters of alpha particles, gives a faster response, attaining relaxation times of the order of a few seconds. Certain mechanical collectors have relaxation times of the order of a few hundredths of a second.
Chalmers, J. A. 1957. Atmospheric Electricity. 84–92.