9# Prof. Rutherford on Excited Radioactivity and 



(3) Excited radioactivity can be produced on bodies i£ the 

 emanation and not the radioactive substance itself is 

 present. Jt can be produced at long distances from the 

 radioactive compound by blowing the emanation mixed 

 with air along tubes. In the case of radium, the emana- 

 tion, which has been introduced into a vessel bv blowing 

 a current of air over the active substance, produces ex- 

 cited activity after a month's interval, although the 

 radioactive substance itself has not been placed in the 

 neighbourhood. On the other hand, the power that a 

 thorium or radium compound has of producing excited 

 activity on a body near it is almost completely lost by 

 blowing over the compound a current of air which 

 removes the emanation as rapidly as it is formed. 

 The amount of emanation or excited activity has no direct 

 connexion with the radioactivity of the compound in its 

 neighbourhood, and cannot be ascribed to any action of the 

 " straight line " radiation in the gas through which it passes. 

 For example, deemanated thoria produces only a small 

 fraction of the amount of excited activity of an equal weight 

 of ordinary thoria although the amount of the straight line 

 radiation is not much affected by the process of deemanatiom 



§ 3. Method of Transmission of Excited Activity. 



The characteristic property of excited radioactivity is that 

 it can be confined to the cathode in a strong electric field. 

 It is probable, therefore, that the radioactivity is due to the 

 transport, in the electric field, of positively charged carriers 

 of some kind. Experiments were undertaken to test this 

 and to find the rate at w T hich these carriers moved in an 

 electric field, in order to obtain a rough estimate of their 

 dimensions compared with a gaseous ion. 



The method employed to determine this velocity is a modi- 

 fication of one already used in a determination of the velocity 

 of the negative ion, produced at the surface of a metal by 

 ultra-violet light*. It depended on the use of an alternating- 

 electric field. By means of a revolving commutator, a direct 

 P. D. was commuted into an alternating P. D. of known 

 frequency. If such an alternating field is applied to two 

 parallel plates, between which a radioactive emanation is kept 

 uniformly distributed, equal amounts of excited activity are 

 produced on each electrode. > ^mHa* 



If in series with an alternating P. D. +E a battery is 

 placed of E.M.F. E, less than E , the positive carrier moves 



* Rutherford, Proc. Camb. Phil. Soc. 1897. 



