5.34 Dr. A. S. Eve on the Number of Ions produce 'd by 



Rutherford & Boltwood standard at McGill University, and 

 with another standard which was tested at Manchester 

 University by the kindness of Professor Rutherford. 



The mean result thus obtained was K' = 3'74x 10 9 , with a 

 possible error of about 5 per cent. Thus the number of ions 

 produced by the j rays from 1 curie of radium C at 1 metre 

 from the source in air, at atmospheric pressure, is 3'74x 10 5 

 per cm. 3 per second *. 



In the Phil. Mag. Sept. 1906, is an account of a previous 

 determination of this constant, and the value for K' then 

 found with an aluminium testing vessel, for pure radium 

 bromide, with £ = 3'4xl0 10 , was 3'1 x 10 9 . This gives for 

 radium, with e = 4'9xl0 10 , K' = 3*68x 10 9 . This is 2 per 

 cent, less than the present determination. I have placed 

 aluminium cylinders about 1 mm. thick over the electro- 

 scope used in the experiments now described, and find that 

 with 7 rays alone there is an increase of about 5 per cent, in 

 the ionization in the electroscope due to the surrounding 

 aluminium cylinder, provided ft rays are not coming from 

 the source. The agreement between the old and new 

 determinations is not, therefore, so good as at first appears. 

 The former work, however, involved the very uncertain 

 correction for a considerable absorption by lead, an error 

 which has been now obviated. 



The total number of ions per second due to Q curie of 

 radium C is, therefore, in air 



Jo r 2 p 



On putting Q=l, it follows thatN', the total number of ions 

 per second produced in air by the 7 rays from one curie of 

 radium C, is 



N , = 47rx3-74xl0 9 /'000044 = l-06xl0 15 . . (2) 



Owing to uncertainty in the value of fi', this may have an 

 error as large as 30 per cent. 



In the work described above, the method of determining 

 the capacity appeared open to criticism, although I believe 

 that errors were eliminated in consequence of the number 

 and variety of wires and spheres employed in the deter- 

 mination. 



Professor H. A. Wilson suggested to me the use of an 



* In a paper to the Phil. Mag. Jan. 1911, I took K'=3'lxl0 3 instead 

 of 3*74 xlO 9 . All the results referring to y.rays in that paper require 

 multiplication by 1*2. 



