DIELECTRIC CONSTANTS 53 



There is a constant relation between the charge of a body and 

 its electric potential. This ratio is called the capacity of the 

 body as a conductor, and depends on the nature of the medium 

 separating the two charged bodies . The tendency of two oppositely 

 charged bodies to come together is altered by altering the dielec- 

 tric or medium separating them. Air is taken as the standard 

 dielectric, and all other dielectric constants are referred to that 

 of air as unity. From the accompanying table it will be seen 

 that water has a high dielectric constant. 



Air =1-0 

 C0 2 =1-0004 

 H =0-9997 



(The determination of the dielectric constant of different 

 substances is of interest from its bearing on Maxwell's electro- 

 magnetic theory of light. The dielectric constant of a substance 

 in the case of light of long wave-length is the same as its refractive 

 index squared.) 



According to the electron theory, an atom is composed of 

 electrons. Electrons are all similar, and are supposed to be not 

 sensible matter, but the smallest possible unit of negative elec- 

 tricity Atoms of different substances owe their different qualities 

 to the varying number of electrons they contain and to the 

 diversity of their combination. These electrons are supposed to 

 exercise an obstructing influence on the passage of an electric 

 charge due to their tendency to move in the direction opposite 

 to the direction of the current. The larger the number of the 

 electrons, therefore, the greater the obstruction and the greater 

 the value of the dielectric constant. Water has a high dielectric 

 constant, and so once the molecule has become dissociated, the 

 resultant ions may approach very near to one another without 

 combining. In short, the dielectric constant is a measure of the 

 insulating capacity of the substance. The addition of solutes 

 alters the value of the dielectric constant of the solvent. 



The atoms in a molecule are held together by very strong 



