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.