  We present a simple analytic gravitational solution which describes the
holographic dual of a 2+1-dimensional conductor which goes beyond the usual
linear response. In particular it includes Joule heating. We find that the
nonlinear frequency-dependent conductivity is a constant. Surprisingly, the
pressure remains isotropic. We also apply an electric field to a holographic
insulator and show that there is a maximum electric field below which it can
remain an insulator. Above this critical value, we argue that it becomes a
conductor due to pair creation of charged particles. Finally, we study 1+1 and
3+1 dimensional conductors at the nonlinear level; here exact solutions are not
available and a perturbative analysis shows that the current becomes time
dependent, but in a way that is captured by a time-dependent effective
temperature.
