A modified theoretical model on the primary electroviscous effect, based upon the inclusion of the condition of that the ions within the Stern layer are allowed to move tangential to the surface of the particle, is proposed. Booth's analysis (Booth 1950a), valid for spherical particles at low zeta-potentials, is the start point, and the effect of a dynamic Stern layer is included. The analytic results are compared with full numerical computations and a good agreement is found for zeta-potentials less than 25 mV.