140 STATIC ELECTRICITY 



The difference in the values for K T in the earlier and later 

 experiments is probably to be ascribed to slight differences in the 

 constitution of the specimens. 



General expression for the resultant force on a surface 

 due to the electric tension and pressure. Let a rectangular 

 tube of strain meet the surface in AB, Fig. 92, making 6 with the 

 normal, and let the area of AB be a. Consider the equilibrium 



of the wedge with section ABC. 



2^1)2 

 Perpendicular to BC we have a pull .,. a cos #, since the 



area of BC is a cos 0. 



Perpendicular to AC we have a push ^ a sin 0. 



The forces on the sides parallel to the paper neutralise each 

 other. Resolving along the normal and tangent, we have 

 Along the normal outwards 



a (cos 2 sin 2 0)= 



a cos 



.. - T r 



K K 



Tangential along AB, 



2-TrD 2 2 1 ) ; 



-^ a (cos 6 sin + sin cos 0)=" .. >in20. 



The force on AB must be equal to the resultant of these, sincv 



P 



FIG. 92. 

 the wedge considered is in equilibrium. 



wards per unit area, and at an angle 



p 



FIG. 93. 

 Thus there is a force out- 



with the normal. 



The system of forces thus made up should give as resultant the 

 force actually observed in any case. 



The electric stress system is not an elastic stress 

 system, and is not accompanied by ordinary elastic 

 strains. The electric stress system applies equally whether the 



