Positive characteristics have been found for diffuse, striationfree, 1-20 ma dc, positive columns in 150-mm Xe, Kr, or Ar, with 0.1% ${\mathrm{N}}_{2}$ in 10-cm tubes. With Xe+${\mathrm{N}}_{2}$, a continuous spectrum is emitted; with other rare gases selections of ${\mathrm{N}}_{2}$ bands. The positiveness of the characteristic results from (1) the disappearance of ions principally by dissociative recombination and (2) ionization which is in effect single stage. For similar current densities the gradient varies little with tube diameter. Probe measurements indicate that the electron and ion density has a relatively flat distribution over a large central part of the tube as compared with the Bessel distribution; the latter resulting from ion loss to the walls by ambipolar diffusion. This broadening out of the discharge increases with the current. For steady convection-free discharges where volume recombination and diffusion are both contributing to ion loss, the degree of constriction should be governed by a "constriction number" $C$ given by $C=\frac{{D}_{a}}{\ensuremath{\alpha}{R}^{2}{n}_{e}}$, where ${D}_{a}$ is the ambipolar diffusion constant, $\ensuremath{\alpha}$ is the recombination coefficient (supposed constant everywhere), $R$ is the tube radius, and ${n}_{e}$ the ion concentration.When the current is increased beyond a critical value, the discharge changes abruptly to a filamentary form, the same whether ${N}_{2}$ is present or not, having a several-fold lower gradient, a negative characteristic, and emitting mainly the line spectrum of the rare gas. This discharge is believed to be diffusion and convection controlled, the (atomic) ions diffusing from the hot core to the cooler periphery where they form molecular ions and recombine dissociatively.Such a mechanism as the above, involving a temperature gradient favoring the existence of molecular ions and dissociative recombination in the outer regions is believed to account quite generally for constriction greater than that corresponding to the Fabrikant-Spenke curve.