Probe measurements were made in the plasma of a low pressure mercury arc. The electron‐energy distributions showed depletions from a Maxwellian distribution in the high energy range. Coupling effects between adjacent probes were investigated and were found to be quite small but in the proper direction to agree with the Langmuir‐Tonks theory. Drift‐current distortion of the random electron‐energy distributions was measured with a bidirectional probe and compared with theory. A multisection probe extending from tube axis to tube wall allowed a determination of radial potential and density variations. Results over a pressure range from 3.4 microns to 35 microns showed good agreement with the ambipolar diffusion theory based on cumulative ionization. A direct calculation of ionization rate in the plasma was made from the ionization probability for a one‐step ionizing process; comparison of this calculation with the observed ionization rate at 1.7 microns indicated that at that pressure the ionization is half direct, half cumulative. For higher arc pressures cumulative ionization evidently predominates.