CH. xxi.] VALIDITY OF OLD VIEWS 197 



field varies inversely as the square of the distance 

 from the moving charge ; and their vector product is, 

 as regards direction, perpendicular to the radius 

 vector at any point. It is proportional, at ordinary 

 speeds, to the sine of the angle between the radius 

 vector and the direction of motion ; while in magni- 

 tude it falls off as the inverse fourth power of the 

 distance. All this can be realised by common sense 

 with very little trouble. 



So, then, take a moving electron, and consider the 

 distribution of its momentum in the space round 

 it. Between its surface and a space of a hundred 

 times its diameter, 99 per cent, of its momen- 

 tum is contained ; because, to reckon it, we should 

 have to integrate the factor 



But a hundred times the diameter of an electron 

 is only 10~ n centimetre, that is to say, the thousandth 

 part of the diameter of an atom. So, within the 

 boundary of an atom, which is a hundred-thousand 

 times an electron's diameter, there is practically none 

 of its momentum not included. 



And even in one of the comparatively closely 

 packed atoms, e.g. in a platinum or mercury atom, 

 the overlapping of momentum for each constituent is 

 extremely small, since their average space apart / 

 is some thousand times the size of each constituent 

 electron. 



Consequently the assertions that an electric current 

 is a transfer of electrons, and that the energy of a 

 current travels in the space surrounding the moving 

 electricity, are statements not inconsistent with each 

 other. Nor are the statements inconsistent that the 



