It is usually assumed that there must be particle-particle collisions or wave-particle collisions (noise or radiation) in order to have a finite electrical conductivity. A finite conductivity is necessary for the reconnection of field lines at a magnetic neutral point. The above assumption would therefore imply that some sort of collisions are necessary for reconnection. Dungey has argued that collisions are not necessary to have a positive value of E · j (dissipation of energy), using an analogy with a diode. Indeed, for certain plasma geometries, an effective conductivity may be defined when the characteristic system length is small compared to a collisional mean free path. For the current sheet in the geomagnetic tail, the effective conductivity may be eleven orders of magnitude smaller than the collisional conductivity. The effective conductivity is essentially determined by the inertia of the particles, and therefore reconnection can proceed without collisions, with energy being carried off by accelerated particles. Inertial and gyro-conductivities and a new ‘current sheet adiabatic invariant’ are defined.