Diurnal‐variation observations of the concentration of condensation‐nuclei in the atmosphere and of certain meteorological elements have been made simultaneously with those of the potential gradient and of the electric conductivity of the atmosphere. The diurnal‐variation curve for condensation‐nuclei is similar to the general temperature‐curve in that high values occur during the daylight hours and low values during the night. The curve is somewhat similar to that for the potential gradient and in a very general way inverse to that for the atmospheric conductivity. The rate of ionization calculated from the linear recombination‐law, shows a diurnal‐variation curve very similar to that for the negative air‐earth current as computed from the potential gradient and the negative conductivity.The results of this investigation suggest that relative humidity is not a large factor in determining the number of nuclei present in the atmosphere. Calculated values of negative conductivity by means of both the linear recombination‐law and the square‐root law show fair agreement with the observed values of the conductivity—neither shows any decided advantage over the other.Several mechanisms whereby the diurnal variation of potential gradient at Washington may be accounted for through the observed number of condensation‐nuclei, charged or uncharged, are considered, in view of the known universal character of the diurnal variation of the potential gradient, it was considered necessary to explain only a portion of the diurnal variation through some such mechanism. On the basis of experimental results it is shown that this portion may be entirely accounted for through variation in height and in density of a local space‐charge. The necessary variation in height can be satisfactorily explained through the action of local convection‐currents in the atmosphere while the required variation in density can be explained through a variation in the number of condensation‐nuclei.Evidence is presented in support of a theory that the secondary minimum in the potential‐gradient diurnal‐variation curve, occurring at many land stations during the warmest time of the day, is produced through the mechanism of convection‐currents in the atmosphere.