1 86 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



across from plate to plate in the form of a tension of the electric 

 field. Therefore dividing both members of equation (74) by a, 

 we have the tension of the electric field in joule-units of force per 

 square centimeter of section, and in the case of air (k equals 

 unity), we have 



Tension of an electric field in joule-units ") 

 of force per square centimeter of section j 



(76) 



in which /, which is written for E\x, is the intensity of the 

 electric field in volts per centimeter. 



105. Electric potential. The two heavy black circles in Fig. 

 114 represent two long parallel metal cylinders one of which is 



Fie. 114. 



positively charged and the other of which is negatively charged, 

 and the fine curved lines (with arrowheads) represent the lines 

 of force of the electric field in the region between the charged 

 cylinders. The intensity of the electric field at a given point is 

 so many volts per centimeter parallel to the lines of force at that 



