Deposit of Radium in an Electric Field. 443 



in virtue of their charge. When the high potential is positive 

 the activity of the needle falls to a very small value. The 

 recoil atoms which in the absence of an electric field reach 

 the needle by diffusion are now driven back by the electric 



Table III. 



Potential 



in volts. 



Activity of the 

 Needle. 



Activity of the 

 Plate. 



- 800 



100 



100 



+ 800 



45 



05 j 







15 



5-5 



-15000 



3-5 



125 



+ 15000 



•8 



50 



wind, so that the activity of the needle is due only to the 

 recoil from the atoms of emanation in the immediate neigh- 

 bourhood of the needle. The small activity of the needle in 

 this case furnishes strong evidence that there are recoil atoms 

 from the emanation carrying a negative charge. 



Needle electrodes raised to a high potential were exposed 

 also to the uncharged recoil atoms of radium B in the 

 apparatus described in sec. 3. As was to be expected, no 

 traces of activity could be detected on the needle whatever 

 the direction of the electric field. 



Experiments with screened Electrodes. 



8. Further experiments have been arranged which con- 

 clusively prove that the concentration of active deposit on 

 the electrodes is not entirely due to the electric force or to 

 diffusion. The plate exposed to emanation is surrounded by 

 a ring to which small pieces of fine wire are soldered at 

 right angles to the plane of the plate, forming thus an 

 electric screen between the walls of the vessel and the plate. 

 When the plate (and consequently the screen which is in 

 metallic conlact with it) is raised to a potential, the charged 

 carriers of active matter can reach only the wires of the 

 screen and the activity of the plate must remain the same 

 as in the absence of an electric field. Experiments show, 

 however, that the activity of the plate is much larger than is 

 to be expected, and that it increases largely with the amount 

 of emanation used. It is evident that a part of this activity 

 is brought to the plate by some other agency different from 

 the electric force or diffusion. The origin of this part of the 



