376 Mr. E. Edser. An Extension of 



Let us consider a dielectric medium, other than the ether, to be 

 composed of molecules each comprising, in the simplest case, two 

 oppositelj charged atoms at a definite distance apart. The volume 

 actually contained by the atoms is provisionally assumed to be small 

 in comparison with the volume of the interatomic spaces. 



In what follows the" term axis of a molecule will be applied to the 

 vector distance between a negative and its associated positive atom. 

 In an isotropic medium the axes of the various molecules will, in the 

 absence of electrical strain, be inclined indifferently in all directions, 

 so that any element of volume will have no resultant electric moment. 

 If now a difference of potential be established between any two 

 parallel planes in the medium, the positive atoms will all move toward 

 points of lower, and the negative atoms toward points of higher 

 potential. As a result each element of volume will now possess a 

 resultant electric moment. Also, if we define a molecular electric 

 moment as the product of the atomic charge (taken as positive) into 

 the vector distance between a negative and its associated positive 

 atom, the resultant electric moment due to a strained element of 

 volume containing a number of molecules, will coincide in direction 

 with the direction of fall of potential. 



In the unstrained medium we may represent the several moments 

 of the molecules contained in an element of volume by lines radiating 

 uniformly from a point, and ending on the surface of a sphere.* 



To find the electric moment of an element of the strained medium, 

 it is only necessary to determine the sum of the alterations of the 

 component molecular moments in the direction of the fall of potential. 



4. Let P be the electromotive intensity at a point in the dielectric, 

 and let q be the atomic charge, the distance between two associated 

 atoms being I. Then a molecule whose axis makes an angle 6 with 

 the direction of fall of potential, will be subjected to a couple 

 Pql sin tending to decrease & and each atom will be subjected to a 

 force Pq cos 9 tending to increase I. Assuming that the forces of 

 restitution called into play by the displacement of the atoms are in 

 both cases proportional to the linear displacements, we shall have 



1 = KxP 2 sin 0, ^ = K 2 P 2 cos 0, 



where indicates the (infinitesimal) molecular rotation, and JAZ is 

 the linear displacement of either atom in a line with the axis of the 

 molecule. 



The increase of the component molecular moment in the direction of 

 the fall of potential due to these strains, will be 



gZ0 sin + gAZ cos 9. 

 * Maxwell's ' Electricity and Magnetism,' vol. 2, 443. 



