The vertical columnar current density in the global atmospheric electrical circuit depends on the local columnar resistance. A simple model for the columnar resistance is suggested, which separates the local boundary layer component from the upper troposphere cosmic ray component, and calculates the boundary layer component from a surface measurement of air conductivity. This theory is shown to provide reasonable agreement with observations. One application of the simple columnar model theory is to provide a basis for the synthesis of surface atmospheric electrical measurements made simultaneously at several European sites. Assuming the ionospheric potential to be common above all the sites, the theoretical air-earth current density present in the absence of a boundary layer columnar resistance can be found by extrapolation. This is denoted the free troposphere limit air-earth current density, <i>J</i><sub>0</sub>. Using early surface data from 1909 when no ionospheric potential data are available for corroboration, <i>J</i><sub>0</sub> is found to be ~6 pA m<sup>−2</sup>, although this is subject to uncertainties in the data and limitations in the theory. Later (1966–1971) European balloon and surface data give <i>J</i><sub>0</sub>=2.4 pA m<sup>−2</sup>.