Cathode phenomena in glow discharges through iodine vapour†

Cathode sections of glow discharges in iodine, although more stable than the positive column, show pronounced longitudinal contraction and lateral distortion. The cathode glow has the usual form, but incompletely covered cathodes have not been observed. The negative glow has a marked tendency to be concave to the cathode. The Faraday dark space is short. With increase in current, the head of the positive column first projects towards a dimple in the negative glow, and then becomes concave to the cathode and closely wrapped round the negative glow. The glow discharge changes into a filamentary arc for still larger currents.