NEW FACTS ABOUT THE NUCLEUS OF THE 



ATOM^ 



By Gael D. Anderson 

 Norman Bridge Laboratory of Physics, California Institute of Technology 



[With 2 plates] 



No depression has existed during the past 5 years in the world of 

 scientific activity. During this period important advances in the 

 field of " nuclear physics " have come with startling rapidity. Each 

 one of these steps forward has opened up fresh fields for exploration, 

 and successful attempts — made in feverish haste — are daily un- 

 covering new facts and relationships. The frequency with which 

 announcements of new discoveries are appearing almost makes any 

 contemporary survey out of date before it can reach the reader's eye. 



Nuclear physics is a subdivision of the general field of atomic 

 physics. The scientific activities begun before the opening of the 

 present century and continuing up to the present day have given to 

 us a satisfactory and rather complete picture of the atom as a whole. 

 We now know it to be a small complex unit about one one-hundredth 

 of a millionth of an inch in size, containing at its center an exceed- 

 ingly small core or nucleus surrounded by one or more electrons, the 

 number of electrons determining the classification of the atom as a 

 particular element. The electrons which surrounded the nucleus are 

 known as " extranuclear electrons." Research of the past two or 

 three decades has, for example, counted the extranuclear electrons 

 which surround the nucleus in each one of the chemical elements. We 

 know that hydrogen has 1 such electron, helium 2, and so on up to 

 uranium, which has 92. Both the mass and electric charge of the 

 electron have been accurately measured, and the masses of the nuclei 

 of most of the elements have been determined. 



Furthermore, through the cooperation of the mathematical physi- 

 cists and the experimenters in the laboratory, the motions and most 

 of the general properties of these extranuclear electrons can today, 

 in a scientific sense, be said to be understood. 



1 Reprinted by permission from the General Electric Review, vol. 37, no. 12, December 

 1934. 



235 



