Whatever view is taken of the process by which these carriers obtain a positive charge, there can be little doubt that the expulsion of an α particle with great velocity from the atom of the emanation must set the residue in motion. On account of the comparatively large mass of this residue, the velocity acquired will be small compared with that of the expelled α particle, and the moving mass will rapidly be brought to rest at atmospheric pressure by collision with the gas molecules in its path. At low pressures, however, the collisions will be so few that it will not be brought to rest until it strikes the boundaries of the vessel. A strong electric field would have very little effect in controlling the motion of such a heavy mass, unless it has been initially brought to rest by collision with the gas molecules. This would explain why the active matter is not deposited on the cathode at low pressures in an electric field. Some direct evidence of a 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[291] 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. _E₀_ 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. _E₁_ less than _E₀_ 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. [Illustration: Fig. 71.] When the plate _B_ is negatively charged, the E.M.F. between the plates is _E₀_ − _E₁_, when _B_ is positive the E.M.F. is _E₀_ + _E₁_. Let _d_ = distance between the plates, _T_ = time of a half alternation, ρ = ratio of the excited radio-activity on the plate _B_ to the sum of the radio-activities on the plates _A_ and _B_, _K_ = velocity of the positive carriers for a potential-gradient of 1 volt per centimetre.