154 ADVANCED ELECTRICITY AND MAGNETISM. 

 Tension of the electric field in air. Putting k = i and 



writing e for - - in equation (7) of Art. 90 we get: 



(5) 



Now the force F which tends to pull the two plates together 

 shows that the intervening electric field is in tension and the 

 force per unit area is the measure of the tension T. Therefore, 

 dividing both members of equation (5) by a, we get: 



T= ^B' e * ( 



where T is the tension in an electric field of which the intensity 

 is e volts per centimeter. 



Fig. 100. 



Equations (4) and (6) are here derived for a very special case 

 but they give the energy per unit volume and the tension at a 

 point in any electric field in air. 



92. The electric field in the region between two concentric 

 metal spheres. A metal sphere of radius A is surrounded by 

 a hollow metal sphere of internal radius B as shown in Fig. 100. 



