Page:Radio-activity.djvu/342

 process of this character, obtained by Debierne on examination of the excited activity produced by actinium, is discussed in section 192.

191. The following method has been employed by the writer to determine the velocity of the positive carriers of excited activity of radium and thorium in an electric field. Suppose A and B (Fig. 71) are two parallel plates exposed to the influence of the emanation, which is uniformly distributed between them. If an alternating E_{0} is applied between the plates, the same amount of excited activity is produced on each electrode. If, in series with the source of the alternating, a battery of E_{1} less than E_{0} is placed, the positive carrier moves in a stronger electric field in one half alternation than in the other. A carrier consequently moves over unequal distances during the two half alternations, since the velocity of the carrier is proportional to the strength of the electric field in which it moves. The excited activity will in consequence be unequally distributed over the two electrodes. If the frequency of alternation is sufficiently great, only the positive carriers within a certain small distance of one plate can be conveyed to it, and the rest, in the course of several succeeding alternations, are carried to the other plate.

Fig. 71.

When the plate B is negatively charged, the between the plates is E_{0} - E_{1}, when B is positive the is E_{0} + E_{1}.