Dependence of the transition from Townsend to glow discharge on secondary emission

In a recent paper, Šijačić and Ebert [Phys. Rev. E. 66, 006410 (2002)] systematically studied the transition from Townsend to glow discharge, referring to older work by von Engel and M. Steenbeck [Elektrische Gasentladungen. Ihre Physik und Technik (Springer, Berlin 1934), Vol. II] up to Raizer [Gas Discharge Physics (Springer, Berlin, 1991)]. Šijačić and Ebert stated that this transition strongly depends on secondary emission [Formula presented] from the cathode. We show here that the earlier results of von Engel and Raizer on the small current expansion about the Townsend limit actually are the limit of small [Formula presented] of the Šijačić and Ebert expression, and that for larger [Formula presented] the old and the Šijačić and Ebert new results vary by no more than a factor of 2. We discuss the [Formula presented] dependence of the transition, which is rather strong for short gaps.