Electrical stability of bulk "S-shaped" negative differential conductivity media

As originally postulated by Ridley, bulk media exhibiting isothermal current-controlled negative differential conductivity (NDC) are shown to be unstable with respect to small fluctuations in the current density, leading to current density filamentation. In contrast to Ridley´s arguments, however, we describe and elucidate the instability in terms of internal-induced magnetic flux. We show that Lenz´s law does not apply to NDC points.