ELECTRIC FIELD. 141 



If the lines of force in an electric field are like stretched fila- 

 ments of rubber one would expect the lines of force to pull 

 outwards on every part of the surface of a charged body. In 

 fact, each part of the surface of a charged body is pulled out- 

 wards by the surrounding electric field. This outward pull 

 may be beautifully shown by pouring melted *rosin in a thin 

 stream from a metal ladle which is supported by an insulated 

 handle and connected to one terminal of an electric machine. 

 The lines of force which emanate from the lip of the metal ladle 

 pull the melted rosin into extremely fine jets which shoot straight 

 outwards from the lip. These jets congeal in the form of excess- 

 ively fine fibers which float about in the air. 



85. Definition of electric flux in air. Consider an area a at 

 right angles to an electric field of intensity e. The product ae 

 is called the electric flux across the area. Compare this with the 

 definition of magnetic flux in Art. 25. If a is expressed in 

 square centimeters and e in volts per centimeter or abvolts per 

 centimeter, it is evident that ae is expressed in volts X centi- 

 meters or in abvolts X centimeters, respectively. Therefore, in 

 what follows, electric flux will always be expressed in volt-centi- 

 meters or in ab volt-centimeters. 



86. Amount of electric flux which emanates from q coulombs 

 of electric charge. Let us consider the simplest example of 

 electric field, namely, the field between two oppositely charged 

 flat metal plates as shown in Fig. 90. 



(a) This field is uniform, that is, the lines of force are straight 

 and parallel. 



T? 



(b) The intensity of the field is , and this expression for the 



oc 



field intensity holds true whatever the nature of the dielectric, 

 whether it be air, or oil or glass. 



(c) The electric field is to be thought of as directed away from 

 the positively charged plate and towards the negatively charged 

 plate; therefore: 



LTY OF AP , LiED sc,=N Ce . '< 



Dat. ' 



