WHEN radioactive contamination occupies an extended area, as, for example, the surface of laboratory benches, its determination by the usual method of β-ray counting is tedious and is relatively not precise. However, the activity can be assessed in this case by a very simple, fast and sensitive method —the discharge of a conductor. The loss of charge of a conductor is due to the aerial ions of opposite sign generated by ionizing radiations, and the method of the discharge of a conductor has formerly been used for routine measurements of atmospheric conductivity1. I can give here only a very brief account of this method, which is based on the relation between the charge Q on a conductor and rate of loss of charge, given by Coulomb2: It can be easily shown that the constant a of Coulomb's equation is equal to 4πnw, n being the number of ions per cm.3 of air surrounding the conductor and w their mobility in the electric field. From (1) we finally get the relation: where K is a constant implying the capacity constants of the measuring device, and t the time which corresponds to the drop of potential from V 0 to V t. The validity of (2) supposes that V 0, V t and w do not vary during the experiment, a condition practically always fulfilled for laboratory measurements.