11] IONIZATION THEORY OF GASES 63 
is lonized in the space between them, If the gas is uniformly 
ionized between two parallel plates, Child and Zeleny have shown 
that there is a sudden drop of potential near the surface of both 
plates, and that the electric field is sensibly uniform for the inter- 
mediate space between them. The disturbance of the potential 
gradient depends upon the difference of potential applied, and is 
different at the surface of the two plates. 
In most measurements of radio-activity the material is spread 
over one plate only. In such a case the ionization is to a large 
extent confined to the volume of the air close to the active plate. 
The potential gradient in such a case is shown in Fig. 9. The 
dotted lme shows the variation of potential at any point between 
the plates when no ionization is produced between the plates; 
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curve A for weak ionization, such as is produced by uranium, 
curve B for the intense ionization produced by a very active 
substance. In both cases the potential gradient is least near the 
active plate, and greatest near the opposite plate. For very 
