At gas pressures less than the Paschen critical value the sparking potential rises rapidly with decrease in pressure. Measurements are difficult because of undesired discharges over longer paths than the intended one. A sparking tube has been constructed with which measurements have been made on air, carbon dioxide, hydrogen, and helium up to 24 kv with nickel electrodes, and on air with a stainless steel cathode. The results are compared with those of Carr, of Cerwin, and of Penning. The equations of the straight portions of the sparking potential curves are of the form $V=a+blog\mathrm{pd}$. From these equations the number of electrons released by the cathode per positive ion impact has been calculated by a method described by Dempster.

• Received 11 January 1939

DOI:https://doi.org/10.1103/PhysRev.55.482