198 ELECTRICAL PHENOMENA [CH. xxi. 



mass of a body resides in its atoms, and that inertia 

 or momentum is a property due to the self-inductive 

 influence of the electromagnetic field surrounding a 

 moving electric nucleus. 



The same sort of thing may be said of the way in 

 which a current is propelled. The pace of progression 

 of electrons through a solid may be considerable, see 

 next section, but it is very far below the pace at which 

 a telegraphic signal travels along a wire. They must 

 be propelled by a lateral action, transmitted through 

 the ether with the speed of light appropriate to the 

 surrounding insulator, by some arrangement which 

 "Modern Views" symbolised in the form of cog- 

 wheels : they cannot be impelled by end thrust. The 

 vf electric current is a more material entity, or has a 

 more nearly material aspect, than was thought 

 probable a little while since ; but all that was taught 

 about its mode of propulsion, and the diffusion of the 

 propelling force from outside to inside, through 

 successive layers, as it were, of the wire all that 

 was taught about the paths by which the energy 

 travels and arrives at point after point of the 

 conductor, there to be dissipated as heat, remains 

 true. 



Number of Ions in Conductors. 



The immense number of electrons that are necessary 

 to make up the mass of a piece of platinum, or of a 

 lump of matter like the earth, can readily be 

 estimated ; so, also, it is easy to imagine . that an 

 enormous number must be travelling in order to give 

 customary strengths of current such as can readily 

 pass through a liquid. 



Through a gas, a limit is soon found to the available 

 number ; and accordingly the conductivity of an 



