Atomic Nucleus and the Law of Force. 743 



The numbers in brackets denote the number of scattered 

 particles counted in the experiment. 



The weighted mean of these results is 77*4, while the 

 atomic number of platinum is 78. The accuracy of the 

 determination depends chiefly on the number of particles 

 counted, for all other quantities are given by measurements 

 of mass and length. If a number Z of a particles is counted, 

 the error due to probability fluctuations is \/Z, i. e. the 

 relative error is l/\/Z. Since about 3700 scattered particles 

 were counted and roughly 8000 direct particles, the probable 

 error in the ratio is about 2^ per cent. As N is proportional 

 to the square root of the ratio; the error in the value of the 

 nuclear charge will be a little over 1 per cent. 



Silver. — -The foil had a weight of 4*50 mgm. per sq. cm., 

 giving 



irf = 2*50 Xl0 19 



and the average 



^ = 7-05xl0 14 . 

 Jtti 



The following values of N were obtained : 



46-0 (453), 41-0 (156), 44-0 (190), 44-0 (307), 45-9 (529), 

 40-5 (411), 491 (224), 46'6 (416), 49-1 (297). 



The weighted mean is 46*3 and the total number of 

 scattered particles is 3000, giving a probable error in N 

 of lj? per cent. 



The atomic number of silver is 47. 



Copper. — The foil weighed 4*78 mgm. per sq. cm. 

 Hence 



^ = 4-64 xlO 19 

 and the average 



m |- 2 =6-85xl0 14 . 



The following values were obtained for N : 

 27'8 (12£), 29-0(340), 29-6(369), 27-6(212), 29-0(178), 

 29-6 (364), 30-0 (377), 28'2 (243), 30'4 (351), 29 6 (302). 



The weighted mean is 29*3 and the total number of 

 scattered particles 2900, giving a probable error in N 

 of about H per cent. 



The atomic number of copper is 29. 



