238 PHYSICAL SCIENCE 



the electrons occupy an exceedingly small fraction 

 of the whole volume of the atom, just as the 

 planets occupy a very small fraction of the space 

 comprised within their orbits. 



The mass of the electron being electrical in its 

 nature, we may calculate the size of the individual 

 electrons or corpuscles from the expression 2^73^ 

 for the electrical mass. We know the values of 

 e and of elm, and from these results we calculate 

 a to be about io~^^ centimetre. According to 

 Thomson, a is the radius of a sphere outside 

 which the momentum of the electric field exists. 

 It seems reasonable to identify this sphere with 

 the effective dimensions of the electron itself. 



We have already seen that, in a substance like 

 water, where the molecules are packed fairly closely, 

 I cubic centimetre contains about 3 x 10^^ mole- 

 cules, or, let us say, 10^^ atoms. Along each edge 

 of the centimetre cube about 4X 10^ atoms are 

 ranged, and thus we may take the effective radius 

 of an atom to be about 5 x io~"^ of a centimetre. 

 Its volume would be about io~^^ of a cubic centi- 

 metre, while the volume of an electron, according 

 to the above estimate of the radius, is about 

 4 X 10""^^ Thus, while the diameter of an electron 

 is less than the hundred-thousandth part of that 

 of an atom, the volume of an electron is only 

 about the lo"^*^ part of that of an atom, and their 

 relative sizes might be compared by the Illustration 

 of a fly roaming about Inside a cathedral. 



On the planetary theory of the atom, the 

 moving electric charges produce a magnetic field, 

 just as does a current flowing round the coils of 

 a galvanometer. Thus, conversely, an impressed 

 magnetic force should modify the movement of 



