55G Er. A. S. Eve on the Number of Ions produced by 

 every other part o£ the apparatus was earthed. The charge 



o 



ITT" 



on the central wire and disk was, therefore, -. — . V, where r 



9 bird ' 



is the radius of the disk, and d its distance from the lowest 

 plate. The disk and wire were then insulated, and all parts 

 of the apparatus earthed. The charge on the central system 

 and disk was now CV, where C is the capacity to he deter- 

 mined and V the potential found by the position of the gold- 

 leaf and from the calibration curve. In the apparatus 

 employed r=5'4 cm., d=l'5 cm., V/V' = 3*25, whence 

 C = 15-8. 



At 91 cm. the 7 rays from 8*6 mg. Ra caused an ionization 

 current of 13"5 volts/min., and as the volume of the electro- 

 scope was 6100 cm., </ = 3960. In this part of the experi- 

 ment, it is necessary to surround the two lower aluminium 

 plates with a collar of thin mica, shown in the figure. 

 Therefore 



3960 = K'x-0085/91 2 , 

 so that 



K' = 3-81xl0 9 . 



This is about 2 per cent, greater than my previous value, 

 3'74xl0 9 . Although the capacity is determined more 

 accurately with the guard-ring and disk electroscope, it pro- 

 bably yields too large a value of Knowing to some secondary 

 radiation from the aluminium plates. The results, gave, 

 therefore, a satisfactory verification of the first set of 

 determinations. 



Number of Ions due to ft rays. 



When an attempt is made to determine the values of 

 quantities corresponding to K' and W , but relating to ft rays 

 instead of 7 rays, difficulties occur. If the radium is placed 

 in a receptacle, the walls absorb some of the ft rays, and if 

 the. radium is placed on a plate, there is 6i reflected " radiation 

 from the plate. The method employed to overcome these 

 difficulties was as follows : — 



The number of ions per cm. 3 per second, produced by the 

 ft rays from Q curies of radium C, at a distance r, is given by 



where K is the constant for ft rays and fi is the coefficient 

 of absorption by air of the 3 rays. 



