MODERN PHYSICS 17 



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 electro magnetic field can travel forward, then fur- 

 ther 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 

 o.i 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 elec- 

 tromagnetic laws. Therefore, we have good reason for 

 assuming that the negative electron is nothing but a dis^ 

 embodied electrical charge, and that its inertia is wholly 

 of electrical origin. 



With respect to the positive electron, we have not such 

 convincing evidence as yet, but it is obviously in the in- 

 terest of simplicity to assume one kind of inertia rather 

 than two kinds. Further, we have a little bit of evidence 

 of this kind, and I wish to mention what it is, because that 

 will furnish an introduction to my sixth important mod- 

 ern discovery. We have good reasons for thinking 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 we never find it 

 any less. If this inertia is all electrical^ then we know 

 from theory that the charge must be more condensed in 

 the positive than in the negative ; consequently, if we are 



