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 line shows the variation of potential at any point between the plates when no ionization is produced between the plates; curve A for weak ionization, such as is produced by