108 



or 



STATIC ELECTRICITY 



D x a cos 1 = D 2 a cos 6 t 

 Dj cos Oi = D 2 cos 2 



or the normal component of the strain is the same in each 

 medium. 



If we are considering non-crystalline media, in which the 

 directions of strain and intensity coincide, let the plane of Fig. 74 

 pass through AC and the normal; the tube in the second medium 

 must also be in that plane. For the components of the intensities 

 at C in any direction parallel to the surface are the same in both 

 media. The intensity in the first medium has zero component 

 perpendicular to the plane of the figure. Therefore the intensity 

 in the second medium has zero component in that direction. ThU 

 implies that CE lies in the plane of the figure or the plane through 

 AC and the normal. Then the incident and refracted tubes are 

 in the same plane with the normal. 



The continuity of potential gives us 



E 1 sin ^! = K 2 sm #r 

 The continuity of strain gives us 



whence 



or, since 



x cos 1 = D 2 cos 



We shall now consider some effects of the presence of a dicloc trie- 

 other than air, which will be useful when we come to dcM rilx- 

 methods of measuring specific inductive capacity. 



Capacity of a condenser with a slab or plate of 

 dielectric inserted. Let us suppose that the distance bet" 



FIG. 75. 



the charged conducting plates is d, and that a slab of dielectric 

 with constant K occupies a thickness t of the space between the 

 plates and that its surfaces are parallel to the plates as in Fig. 75. 

 Let the plates and slab extend indefinitely. Let the surface density 



