CH. ii.] ELECTRIC INERTIA 15 



or not there really exists any other kind of inertia. 

 The question whether there does or not is at 

 present, strictly speaking, an open one. 



Effect of Concentration. 



The only way of conferring upon a given electric 

 charge any appreciable mass, is to make its potential 

 exceedingly high, that is to concentrate it on a 

 very small sphere. 



A coulomb at the potential of a volt has an elec- 

 trostatic energy of half a Joule, that is ^ x 10 r ergs. 



The mass equivalent to this would be 



2 10^ 2 



39xl0 20 = 27 X 10 ~ 13 g ramme= 10 ~ 8 milligramme. 



Kaise the potential to a million volts, and the 

 mass-equivalent to a coulomb at that potential 

 would be the hundredth part of a milligramme ; 

 still barely appreciable therefore. 



The charge on an atom as observed in electrolysis 

 is known to be 10~ 10 electrostatic units.* If this 

 were distributed uniformly on a sphere the nominal 

 size of an atom, viz., one 10~ 8 centimetre in radius, 

 its potential would be one hundredth of an elec- 

 trostatic unit, or . about 3 volts. The energy of 

 such a charge would be 10~ 12 erg, and the inertia 

 of a body which would possess this energy if moving 

 at the speed of light would be 10~ 33 gramme ; which 

 would therefore be its electrical inertia or extra mass. 



But this is incomparably smaller than the mass 

 of a hydrogen atom, which is approximately 10~ 25 

 gramme. Consequently the ionic charge distributed 

 uniformly over an atom would add no appreciable 

 fraction to its apparent mass. 



* More exactly, according to Cambridge measurements, 3*3 x 10 ~ 10 . 



