WHAT IS ELECTRICITY? — HEYL 229 



quite another thing. Freedom of motion of the lines of force in such 

 a structure must be almost nonexistent. And if we make the alter- 

 native assumption that the neutron is an independent, nonelectrical 

 entity, the electrical theory of matter must admit of an important 

 exception. 



But an electrical theory of matter to be acceptable must admit of 

 no exceptions. It must obey the " all or none principle." If it is 

 approximate in even the slightest degree we are confronted with the 

 existence of two kinds of matter, ordinary and electrical, and we are 

 violating the rule of simplicity in reasoning laid down by Newton. 



But has there not been later evidence supporting this theory? 



It has sometimes been said that Millikan's oil-drop experiments, by 

 which he measured the charge on a single electron, prove the con- 

 stancy of this charge, and hence the variability of the mass alone in 

 Kaufmann's experiments. It is true that Millikan found that the 

 charge on an ion after it had been transferred to the oil drop was the 

 same whatever the source of the original charge. Ions of different 

 gases, imquestionably of different speeds, gave the same charge to 

 the drop. But it is to be remembered that the measurement of this 

 charge was made not at the speed of the ion but at that of the oil drop, 

 which was of the order of a few hundredths of a centimeter per 

 second. 



The special theory of relativity is sometimes quoted in support of 

 the constant charge and variable mass. It is true that Einstein ^^ in 

 his original paper of 1905 gives a formula for the change of mass 

 with the speed of a moving electron, which, like J. J. Thomson's 

 formula, becomes infinite at the speed of light, and that he gives no 

 similar formula for a change in the charge. It will be interesting 

 for us to see how he obtained this result. 



In section 10 of his paper Einstein derives the following formula 

 for the a7-component of the acceleration of a moving charged par- 

 ticle, together with formulas for the other components : 



(Px _ _£ JL XT 



in which e is the charge on the particle, m its rest mass, X the com- 

 ponent of the electric vector, and ^ the familiar 1 /-y/l-v'^/c-. 



It is evident that the quantity e/m is altered by the factor l/yS^, 

 but whether the charge or the mass or both are changed is not obvi- 

 ous. Einstein without comment assumes e to be constant and m to 

 bear the full effect of the modifying factor, and on this basis derives 

 his formula for the change of mass. 



« Einstein, Ann. Phys., vol. 17, p. 891, 1905. 



