Electrical Fields in the Crookes Dark Space of a Low Pressure Glow Discharge in Air andN2

Application of a 20 000-volt electron beam probe to the study of the fields in the Crookes dark space of normal and abnormal glow discharges in air and ${\mathrm{N}}_2$ from 50 to 400 microns pressure and current densities from 3 to 20 μa/${\mathrm{cm}}^2$ were made. In general agreement with Aston's low pressure study of 1911, the fields generally dropped in a linear fashion from cathode to negative glow. Fields ranged from 600 volts/cm on down at the cathode. Over-all accuracy was 10 percent or better. These fields, together with known variations of ion drift velocity and approximate Townsend second coefficients $\gamma$, allow the ionization per cm along the axis to be computed as a function of position in the dark space. While accuracy of fields at the cathode are not such as to allow of positive statement, the nature of errors present strongly indicate the ionization function in the dark space to be of the type deduced by Morton under similar conditions in coaxial cylindrical geometry.