228 ANNUAL REPOKT SMITHSONIAN INSTITUTION, 19 3 5 



But much water has run under the bridge since 1893. Forty years 

 is a long life for any physical theory in thege days, and the recent 

 discovery of the neutron has brought with it a challenge to the 

 electrical theory of matter. 



In J. J. Thomson's original theory of the increase in mass of a 

 moving charge it was an essential point that the lines of force should 

 be free to adjust themselve3 as the motion demanded. As a leaf or 

 a card tends to flutter down through the air broadside on, so the 

 lines of iorce, originally distributed radially and symmetrically 

 about the charge at rest, will tend to set themselves in a plane 

 perpendicular to the direction of motion of the charge. They will 

 not all be able to lie in this plane because of their mutual repulsion, 

 but the density of the line3 will be a maximum in this plane and a 

 minimum in the direction of motion, and a certain space distribution 

 will result of such a nature that the apparent increase of mass can 

 be completely accounted for. 



But it is essential for this result that the lines of force shall be per- 

 fectly free at their outer ends ; in other words, only a single isolated 

 charge is considered. Now, in a structure like the hydrogen atom, 

 composed of a negative and a positive particle, there is bound to be 

 some interference with this freedom of adjustment. In a neutral, non- 

 ionized atom it would appear that all of the lines must begin and 

 end within the atomic structure. 



J. J. Thomson must be given credit for foreseeing this difficulty, 

 though the Bohr atom was as yet years in the future. He had an 

 atomic concept of his own in mind at that early date and pointed out 

 that the distance betw^een the particles constituting an atom must be 

 thousands of times the diameter of a particle. In consequence, he 

 said, almost all of the mass will originate where the lines have their 

 greatest density, near each particle; and the particles are relatively 

 so far from each other that the parts of the lines of force in their 

 immediate neighborhood will have ahnost perfect freedom of orien- 

 tation with the motion of the atom.^^ 



This is a quantitative question ; but it is clear that only under the 

 most favorable conditions will we have a freedom of motion in the 

 atom which approximates that around an isolated charge, and in con- 

 sequence the electrical explanation of matter on J. J. Thomson's 

 theory must be in the same degree approximate. 



With the neutron, conditions are more rigid. Assuming the neu- 

 tron to consist of a proton and a negative electron, the union of these 

 must be almost as close as possible, as the neutron, on modern theory, 

 may form a constituent of an atomic nucleus. Here we are dealing 

 not with atomic magnitudes but Avith subatomic dimensions, which is 



»8 Thomson, J. J., Electricity and Matter, p. 51, Seribner's, New York, 1904. 



