284 EXCITED RADIO-ACTIVITY [CH. 
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 186. 
185. 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. 54) 





| Es 
H-=-—4Ilk 
| 
| 
B | 
Emanation 
ane 


ieee cl 
Fig. 54. 
are two parallel plates exposed to the influence of the emanation, ' 
which is uniformly distributed between them. If an alternating 
E.M.F. £, 1s apphed 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 is placed of E.M.F. 
E, less than £,, the positive carrier moves in a stronger electric 
fieid in one half alternation than in the other. A carrier con- 
sequently 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 
