The radiation dose rate of the combined natural sources of both cosmic rays and terrestrial radioactivity was measured from the discharge of a negatively charged 10.2 mu m diameter polystyrene particle levitated in an electrodynamic balance, giving good agreement with the predicted natural dose. This measurement was based upon a series of discharge experiments of these particles due to the influence of a cesium 137 gamma ray radioactive source for various cases of amplitude V-ac (800-2000 V) and frequency f (200-700 Hz) of the alternating potential applied to the balance's ring electrode, covering a full range of possible operating V-ac and f values for the balance. These set of measurements revealed an interesting complex behavior of discharge rates. Effects of the initial amount of charge on the particle and the density of gamma rays were also studied as well. A least squares logarithmic second order equation was fitted to these measured discharge data, which easily offered a method of measuring the activity of the natural sources at V-ac=800 V and f=700 Hz. (C) 2004 Elsevier Ltd. All rights reserved.