The ratio L/c has dimensions of length and is a measure of the "effective thickness" of the region at the surface in which the actual concentration is significantly less than the signifi bulk concentration. Indeed, we may consider a very crude picture in which the actual concentration c(x) may be supposed to be zero from the surface inward to a distance xo and to be equal to the bulk concentration c(o) from that distance inward. It will easily be seen from Eq. (4) that