The information that can be obtained about the locations and the magnitudes of currents into active centers in a thundercloud from current density and electric field measurements at the surface of the earth below the cloud are examined. A dynamic interactive electrical model has been used in which the distributed earth, atmosphere, ionosphere circuit is included. In the model, electrification mechanisms in the cloud are simulated by generators; conduction and displacement currents flow in resistors and capacitors between nodes at grid points. Lightning is stimulated by switches which drastically reduce the resistances between nodes when the electric field exceeds an assumed breakdown value and raise them again whe they drop below the value assumed necessary to sustain the arc. The technique allows the divergence of the Maxwell current to be kept zero everywhere. The effects of the intensity and geometry of the generators, the conductivity and conductivity gradient in the cloud, and of the exchange layer and coronal ions are investigated at times when the configurations are reasonably constant, as well as at the beginning and end of a storm. It is shown that the current densities to the ground between lightning discharges provide a good measure of the generator current system associated with the cloud and that their time variations after flashes provide information on the altitude profile of the ccoductivity. As an example of the application of the technique, the currents under a small Florida thunderstorm are analyzed after a cloud-ground discharge.