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CIII. On the Orbits in the Field of a Doublet. By Dorothy 

 Wrinch, Fellow of Girt on College, Cambridge, and 

 Member of the Research Staff, University College, London*, 



Introductory. 



THE present paper investigates in a somewhat systematic 

 way the two-dimensional motion of a particle in the 

 field due to a doublet. Several of the results are of course 

 known already as isolated theorems, but the subject does not 

 appear to have been treated by any writer in a comprehensive 

 manner, and it is very difficult to obtain a general view of the 

 motion of a particle around a doublet even in two dimensions 

 by putting together the specific solutions found in dynamical 

 treatises or memoirs. Some new results which may be of. 

 importance are arrived at in the present paper, for the 

 doublet may be of many types : the analysis is equally 

 appropriate to a magnetic pole moving under the influence 

 of an elementary magnet or to an electric charge in the 

 presence of an electric doublet. Physicists are now generally 

 convinced that the structure of a neutral atom of an element 

 follows the lines generally associated with the names of 

 Rutherford and Bohr. Such an atom behaves towards an 

 external electron sufficiently far away effectively like a 

 doublet, and we have little knowledge of its capacity to 

 attract and retain a stray electron and so form a negatively 

 charged atom. In a vacuum tube, where the atoms are con- 

 tinually bombarded by electrons, we know that negatively 

 charged atoms frequently occur ; but the conditions con- 

 ducive to the presence of a large number in cases in which 

 their existence is chemically possible appear to be quite 

 unknown. 



The success of the quantum theory has generally established 

 the fact that we must not expect to be able, by the use of 

 classical dynamics, to determine the internal motions of the 

 electrons actually constituting the atom ; but if we recall 

 the facts that 



(a) the inverse square law of attraction between 

 electric charges appears to remain valid at distances 

 comparable with atomic dimensions, 



(b) the essential need for the quantum type of 

 dynamical specification only arises when an electron is 

 "bound " or is already a constituent part of the atom, 



we see that the motion of an external charge must, even 



* Communicated by the Author. 

 Phil Mag. S. 6. Yol. 43. No. 257. May 1922. 3 S 



