11] IONIZATION THEORY OF GASES 37 
30. Rate of recombination of the ions. A gas ionized 
by the radiation preserves its conducting power for some time 
after 1t is removed from the presence of the active body. A 
current of air blown over an active body will thus discharge an 
electrified body some distance away. The duration of this after 
conductivity can be very conveniently examined In an apparatus 
similar to Fig. 6. 

A dry current of air or any other gas is passed at a constant 
rate through a long metal tube 7Z. The current of air after 
passing through a quantity of cotton-wool to remove dust particles, 
passes over a vessel 7’ containing a radio-active body such as 
uranium, which does not give off a radio-active emanation. By 
means of insulated electrodes A and Bb, charged to a suitable 
potential, the current through the gas can be tested at various 
points along the tube. 
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 7. 
If the electric field is sufficiently strong, all the ions travel 
in to the electrodes at A, and no current is observed at the elec- 
trode B. Ifthe 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 z 
— = an’, 
dt 
where « is a constant. 
