A “Z‐shaped” current voltage curve is shown to be controllable with a new device which is equivalent to a voltage source in series with a negative resistance, both being adjustable. The latter was built with an operational amplifier. Using this original regulating device, the steady‐state anodic polarization curve of an iron rotating disk in molar sulfuric acid was studied as a function of the rotation speed. The Z‐shaped curve thus obtained exhibits a continuous and reversible transition between active and passive states. This particular shape of the polarization curve explains the steep transitions observed with a potentiostatic regulation. In the passive to active state transition the dissolution of the disk is localized to a ring part of the surface, the dimensions of which were followed as a function of the total current and of the rotation speed. The explanation of this quite heterogeneous distribution of current density can be found in the coupling between the convective‐diffusive mass transfer and a very steep variation of current density vs. potential.