  We study the conductivity in itinerant-electron systems near to a magnetic
quantum critical point. We show that, for a class of geometries, the universal
power-law dependence of resistivity upon temperature may be reflected in a
universal non-linear conductivity; when a strong electric field is applied, the
resulting current has a universal power-law dependence upon the applied
electric field. For a system with thermal equilibrium current proportional to
$T^\alpha$ and dynamical exponent $z$, we find a non-linear resistivity
proportional to $E^{\frac{z-1}{z(1+\alpha)-1}}$.
