Alteration in cell voltage of industrial cells after each feeding of fresh alumina is initially small and accelerates to become extremely fast immediately prior to the incipience of the anode effect. Estimates of the components of the cell voltage on the basis of a mathematical model particularly taking account of the action of the gaseous phase underneath the anode are compared with experimental data from industrial cells. The fundamental agreement (in spite of inevitable model insufficiencies) supports the view that the anode effect is induced as the actual anodic current density approaches the limiting one.