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[1] 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.M.F. 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 E.M.F., a battery of E.M.F. 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.
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Fig. 71.
When the plate B is negatively charged, the E.M.F. between the plates is E_{0} - E_{1}, when B is positive the E.M.F. is E_{0} + E_{1}.
- ↑ Rutherford, Phil. Mag. Jan. 1903.
