Specific Ionization produced by Corpuscles of Radium. 551 



measured by Kaufmann with great accuracy* and found to 

 have all values between 2'36 10 10 and 2*83 10 10 , so that we 

 should expect u for the deflectable Becquerel rays to be less 

 than that for Lenard rays i£ the explanation offered be correct. 

 Townsend states in the paper first mentioned that an esti- 

 mate of a for radium corpuscles was made by him and found 

 to be at least 13 t, a number considerably greater than 0'4. 



In the experiments I am about to describe it will be shown 

 that the number 13 obtained by Townsend is about 76 times 

 too large. 



The radioactive substance used in my experiments was 

 radium chloride obtained from the Societe Centrale de pro- 

 duits chimiques and labelled initial activity 1000. 



The radium J was in the first experiments placed inside a 

 closed vessel and covered with an aluminium plate, the leak 

 being measured between this plate and another above it ; 

 the space surrounding the radium was in connexion with the 

 space between the plates through a small hole in order to 

 equalize the air-pressure in the two spaces. 



With this apparatus the results of the experiments were 

 difficult to interpret owing to apparatus inside becoming 

 radioactive under the action of the " emanation " from the 

 radium. 



The radium was then taken outside and placed about 20 cms. 

 away from the leaking system, so that the emanation could 

 not penetrate to the leaking plates, and also it was certain 

 that a magnetic field in the region in which the radium was 

 placed would not appreciably affect the current between the 

 plates directly. 



It was found that in this position practically all the leak 

 was due to deflectable Becquerel rays or the corpuscles from 

 radium ; the leak due to the corpuscles alone was, however, 

 too small to be measured accurately, and the radium had to be 

 moved closer to the leaking system. 



The final arrangement is shown in fig. 1. One gram of 

 radium was strewn uniformly over the bottom of a lead box 

 B, the top of the box was covered with a thin aluminium leaf 

 I 0'0043 mm. thick in two sets of observations, and with an 

 aluminium plate 0*083 mm. thick in a third. 



Above the radium a thick lead plate PP had a hole 2*5 cms. 

 in diameter drilled in the centre, this was covered on top 



* Nachrichten d. K. Gesell. d. Wissen. Gottingen. 1901. 



f In these experiments his electrometer was not sufficiently sensitive 

 co measure the leak due to the corpuscles themselves, so that a lower 

 limit only could be found. 



X This substance is generally spoken of as radium, though it is really 

 mostly radium chloride! 



