178 ANNUAL BEPORT SMITHSONIAN INSTITUTION, 1918. 



electron is never found, with, an inertia less than the inertia of a 

 hydrogen atom. Let us consider the inertia of the negative. So 

 long as it is moving slowly compared with the speed of light its 

 inertia remains constant because the shape of its electromagnetic 

 field is not appreciably distorted by its motion. But as soon as 

 you imagine it to be moving with a speed which is close to the 

 speed of light, that is with a speed which is nearly as great as the 

 speed with which its own electromagnetic field can travel forward, 

 then further change in speed will distort the field and hence change 

 the inertia. In other words, the inertia of a charge ought to be a 

 function of speed only when the speed approaches the speed of 

 light. As a matter of fact, when it is from 0.1 up to 0.9 of the speed 

 of light, you can compute just how it ought to vary. Now, by some 

 happy chance the physicist has found negative electrons, namely 

 those shot off by radium, which are going with these speeds, and 

 hence it is possible to test our theory for these particles and see 

 whether the rate of change of their inertia with the speed checks 

 with the theoretical value. It is found that there is such a check. 

 This means that there isn't any inertia in those particles which does 

 not obey the electromagnetic laws. Therefore, we have good reason 

 for assuming that the negative electron is nothing but a disem- 

 bodied electrical charge, and that its inertia is wholly of electrical 

 origin. 



Now, with respect to the positive electron, we have not that evi- 

 dence as yet, but it is obviously in the interest of simplicity to 

 assume one kind of inertia rather than two. Further, we have a 

 little bit of evidence upon this point, and I wish to mention what it 

 is, because that will furnish an introduction to my sixth important 

 modern discovery. We have good reason to think, at any rate, 

 that there is only one positive electron in the hydrogen atom, but 

 that the mass, or inertia of that positive is almost' the mass of the 

 hydrogen atom — at any rate Ave never find it any less. Now r if this 

 inertia is all electrical, then we know from theory that the charge 

 must be more condensed in the case of the positive than in that of the 

 negative. Consequently, if we are going to make the observed inertia 

 of the positive hydrogen nucleus all electrical, that nucleus must be 

 an even more dense charge, that is it must be a smaller body than is 

 the negative electron. It was in this way that we first got the pic- 

 ture of an atom which has an extraordinarily minute single positive 

 nucleus, with negative electrons around the outside. But although 

 we first got this picture by that kind of a theory, we do not need to 

 depend upon that theory now, because we know that the conclusion is 

 correct. We know that the atom does consist of a body with a posi- 

 tive nucleus which is extraordinarily minute, and we can tell just 



