150 Rutherford: Modern 



Lorentz, Abraham and others. A moving charge acts 

 like an electric current, and a magnetic field is produced 

 round the body on which the charge is distributed. This 

 implies that magnetic energy is stored up in the medium 

 surrounding the charged body and travels with it. In 

 consequence of this, a charged body moving at a definite 

 speed has more energy associated with it than if it were 

 uncharged. Part of this energy is the ordinary mechan- 

 ical kinetic energy of the body and the rest is electro- 

 magnetic energy due to the charge associated with it. 

 A charged body thus behaves as if it possessed addi- 

 tional or " electrical " mass in virtue of its motion. 

 Theory shows that this electrical mass is constant for 

 slow speeds, but increases rapidly as the velocity of the 

 body approaches that of light. 



Now imagine that we do away altogether with the 

 material sphere, which acts as the carrier of the charge, 

 and consider the motion of a charge of electricity dis- 

 tributed over a small spherical surface but with no mate- 

 rial nucleus. 



This charge of electricity when in motion will be sur- 

 rounded by a magnetic and electric field which travels 

 with it. Energy is consequently associated with it. The 

 moving charge in fact behaves as if it had ordinary mass 

 and has the characteristic property of matter, inertia. 

 It tends to resist any change in the direction or magni- 

 tude of its motion. This mass will be constant for small 



