A gauze screen, placed over the cross-section of the tube at D, serves to prevent any direct action of the electric field in abstracting ions from the neighbourhood of T.
If the electric field is sufficiently strong, all the ions travel in to the electrodes at A, and no current is observed at the electrode B. If the current is observed successively at different distances along the tube, all the electrodes except the one under consideration being connected to earth, it is found that the current diminishes with the distance from the active body. If the tube is of fairly wide bore, the loss of the ions due to diffusion is small, and the decrease in conductivity of the gas is due to recombination of the ions alone.
On the ionization theory, the number dn of ions per unit volume which recombine in the time dt is proportional to the square of the number present. Thus
dn
--- = αn²,
dt
where α is a constant.
Integrating this equation,
1 1
--- – --- = αt,