is
This supposes that the stream of particles passes completely across the vessel. If not, the expression becomes
If the ionization in the vessel AB is measured, and a curve plotted showing its relation to h, the curve in the former case should be a straight line whose slope is nb/ρ and in the latter a parabola.
Thus if a thin layer of radio-active material is employed and a shallow ionization vessel, the ionization would be represented by a curve such as APM (Fig. 40), where the ordinates represent distances from the source of radiation, and the abscissae the ionization