PHYSICS 



experiments by Sir Ernest Rutherford and his 

 collaborators on the scattering of the positively- 

 charged a particles, which are emitted by radio- 

 active substances, in their passage through different 

 kinds of matter. These a particles have been 

 proved to be helium atoms which have lost two 

 negative electrons in other words, the nuclei 

 of the helium atoms moving at very high 

 speeds. These a particles are so minute and their 

 momentum is so great that the majority of them 

 pass through thin layers of various kinds of matter 

 without sensible deflection. Their motion is in 

 fact almost unaffected by the light negative elec- 

 trons they encounter. It is only when they come 

 into relatively close contact with the massive posi- 

 tive nuclei that their paths are sensibly deflected 

 and they are scattered through large angles. A 

 close study of this relatively infrequent large-angle 

 scattering has shown that the law of force between 

 the a particle and the nucleus is the ordinary 

 electrostatic law of the inverse square until the 

 two centres approach within one-millionth of 

 one-millionth part of a centimetre of each other. 

 It has also shown that the electric charge of the 

 nuclei of the different atoms is equal to the charge 

 on the electron multiplied by the atomic number 

 that is to say, by the number which is obtained 

 when the different chemical atoms are arranged 

 in the order of their atomic weights starting with 



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