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V. On the Dynamics of the Electron. By Megh Nad Saha, 

 M.Sc, Lecturer on Theoretical Physics, University College 

 of Science, Calcutta*. 



MASS as a fundamental physical concept has been in- 

 troduced into Physics by Newton's Second Law of 

 Motion, which may be said to form the corner-stone 

 of classical Mechanics. But in spite of its splendid suc- 

 cess, physicists have always encountered some difficulty in 

 realising mass as a fundamental physical concept in the 

 same sense as the concepts of time and space. The funda- 

 mental object of mechanics is to provide a scaffolding by 

 means of which the motion of material bodies can be sur- 

 veyed and followed, when these are subjected to various 

 disturbing influences. Some hypothesis must be introduced 

 for taking into account the influence of thess disturbing 

 agencies. The question is : "Are Newton's Second Law of 

 Motion and the ideas underlying it quite sufficient for all 

 possible cases of motion, or are we to search for some more 

 general principle?" Some physicists are in fact in favour 

 of introducing Energy as a more fundamental physical con- 

 cept than Mass, thereby basing the Science of Mechanics on 

 various Energy-theorems. 



So long as we hold to the principle of invariability of mass, 

 there can of course be no question about the utility of the 

 second law. Bat in the electron we have a physical entity which 

 defies this limitation. If we want to survey its motion, and 

 have no other means of doing so than classical mechanics, we 

 must ascribe to it a certain mass, but for aught we know this 

 mass is neither definite nor invariant during motion. Conse- 

 quently the scaffolding which enables us to study and survey 

 the motion of material (i. e. non-electrical) particles breaks 

 down in this case. Some other system of Mechanics other 

 than Newtonian must be formulated. In this attempt, we 

 must remember that the electric charge is the only invariant 

 physical quantity, consequently in place of mass, this quantity 

 ought to appear in the equation of motion. We must also 

 take cognisance of the newly discovered relations between 

 time and space which are embodied in the Principle of 

 Relativity. 



I may be allowed to remark at this place that though the 

 inadequacy of classical mechanics for studying the motion 

 of electrons is now admitted on all hands, and many attempts 

 are being made for formulating the exact dynamics of the 



* Communicated by Prof. A. W. Porter, F.R.S. 



