The dc sparking potential of pure nitrogen gas has been measured under uniform electric field conditions in the pressure range 15–3000 psi(1–200 atm) and at electrode separations ranging from 0.25–25 mil (6.3–635 &mgr;). Departures from Paschen's law are found to occur at very high fields and to depend upon cathode conditions. A model is proposed in which current is field‐emitted from the cathode under the combined effect of the applied field and a positive‐ion space charge on an insulating layer at the cathode; at sufficiently high fields, the cathode current becomes unstable and breakdown results. The model predicts a relationship between an electron multiplication factor and the applied field, which is found to be consistent with the experimental results.