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 current between the plates _AB_. [Illustration: Fig. 40.] In this case, _PM_ is the range of the α particles from the lowest layer of the radio-active matter. The current should be constant for all distances less than _PM_. For a thick layer of radio-active matter, the curve should be a straight line such as _APB_. Curves of the above character should only be obtained when definite cones of rays are employed, and where the ionization vessel is shallow and includes the whole cone of rays. In such a case the inverse square law need not be taken into account. In the experiments previously recorded (sections 99 and 100), the ionization was measured between parallel plates several centimetres apart for a large area of radio-active material. Such an arrangement was necessary at the time at which the experiments were made, as only weak radio-active material was available. Measurable electrical effects could not then be obtained with narrow cones of rays and shallow ionization vessels, but this disadvantage is removed by the advent of pure radium bromide as a source of radiation. The interesting experiments described by Bragg and Kleeman show that the theoretical curves are approximately realized in practice. The chief difficulty experienced in the analysis of the experimental results was due to the fact that radium is a complex radio-active substance and contains four radio-active products each of which gives rise to α rays which have different ranges. The general character of the results obtained from radium are shown graphically in Fig. 41, curves _A_, _B_, _C_, _D_. [Illustration: Fig. 41.]