742 Mr. J. Chad wick on the Charge on the 



the effective part of the slit was 175*5 mm. Slits of width 

 2*80 mm., 1*24 mm., and '622 mm. were used, giving 

 reduction factors of 197, 445, and SS6. 



The source of u rays was a brass disk of 2 to 3 mm. 

 diameter coated on the face only with Ha (B + C). The 

 initial 7-ray activity of the source was equivalent to about 

 2 mgm. Ra. The preliminary manipulations of inserting 

 the source, exhausting the box, etc., were performed in a 

 weak light, and a further 15 minutes' wait was allowed 

 before counting. 



The scattered and the direct particles were counted 

 alternately. The scattered number was generally about 

 30 to 40 per minute at the beginning, and about 300 

 scattered particles were counted in the course of one 

 experiment. 



In calculating the ratio of the scattered number to the 

 direct number, the correction for decay of the source is 

 comparatively unimportant, since each scattered number 

 is included between two counts of the direct number. 

 From this ratio the value of the nuclear charge of the 

 scattering atom was deduced by substituting the known 

 constants in the expression given in the previous section. 



§4. The Nuclear Charges of Platinum, Silver, 

 and Copper. 



Three series of experiments were carried out, foils of 

 platinum, silver, and copper being used as scattering 

 materials. 



Platinum. — The foil used consisted of 10 sheets of thin 

 leaf in order to ensure a uniform thickness, and had a weight 

 of: 4*93 mgm. per sq. cm. Hence nt==l'52Q x 10 19 . 



Using Geiger's relation for the decrease of velocity in 



passing through the foil, and Rutherford and Robinson's * 



mu wiu* 



value for the initial -=r, the average value of -^- is 



7-20 xlO 14 . h h 



The angles cfo/2 and <£ 2 /2 were 10° 54' and 18° 16' 



respectively. We find, therefore, that the observed ratio. 



of the number of scattered particles per minute to 



the number of direct particles per minute is equal 



toN 2 x3-16xlO" 7 . 



The values of N calculated from the experiments were : 



80-6 (215), 79-4 (226), 79-6 (629), 76-0 (367), 77-1 (184), 78-5 (277), 

 77-0 (162), 76-5 (278), 75-3 (815), 76-5 (345), 76-2 (354). 



* Rutherford and Robinson, Phil. Mag. xxviii, n. 552 (1914). 



