Page:Radio-activity.djvu/360

 or β particles expelled from it per second. In practice, however, this is inconvenient and also very difficult experimentally.

Certain practical difficulties arise in endeavouring to compare the activity of one product with another. We shall see later that, in many cases, all of the successive products do not emit α rays. Some give out β and γ rays alone, while there are several "rayless" products, that is, products which do not emit either α, β, or γ rays. In the case of radium, for example, radium A gives out only α rays, radium B no rays at all, while radium C gives out α, β, and γ rays.

In practice, the relative activity of any individual product at any time is usually determined by relative measurements of the saturation ionization current produced between the electrodes of a suitable testing vessel.

Let us consider, for example, the case of a product which gives out only α rays. The passage of the α particles through the gas produces a large number of ions in its path. Since the α particles from any individual product are projected with the same average velocity under all conditions, the relative amount of the ionization produced per second in the testing vessel serves as an accurate means of determining the variation of its activity. No two products, however, emit α particles with the same average velocity. We have seen that the rays from some products are more readily stopped in the gas than others. Thus the relative saturation current, due to two different products in a testing vessel, does not serve as an accurate method of comparing the relative number of α particles expelled per second. The ratio of the currents will in general depend upon the distance between the plates of the testing vessel, and, unless the relative ionization due to the average α particle from the two products is known from other data, the comparison of the currents can, at best, be only an approximate guide to the relative number of α particles escaping into the gas.

202. Some examples will now be considered to show how the factors, above considered, influence the character of the curves of activity obtained under different experimental conditions. For the purpose of illustration, we shall consider the variation after removal of the excited activity on a body exposed for different times to a constant supply of the radium emanation. The active