364 ELEMENTS OF ELECTRICAL ENGINEERING. 



The electric field. The dielectric. The region between the 

 two bodies A and B, Fig. 3, is of course filled with some elec- 

 trical insulator such as air, or oil, or glass. An insulator between 

 two charged bodies is called a dielectric. This dielectric is the 

 seat of a peculiar stress called the electric field which is similar in 

 many respects to the magnetic field. The lines of force of this 

 electric field trend somewhat as shown in the figure, touching 

 the surfaces of A and B at right angles. These lines of force 

 are thought of as going out from the positively charged body 

 and coming in towards the negatively charged body. 



Electrostatic attraction. The charged bodies A and B, Fig. 

 3, attract each other. This attraction, which is called electro- 

 static attraction, shows that the lines of force of an electric field 

 are in a state of tension and have a tendency to shorten. This 

 tension of the lines of force pulls outwards on the surface of A 

 and of B at each point. This outward pull on the surface of a 

 charged body is very strikingly shown by pouring a viscid liquid 

 over the sharp lip of a charged metal ladle. The liquid is pulled 

 into fine jets by the lines of force which emanate from the liquid 

 as it passes over the lip. When melted rosin is used in this way, 

 the jets congeal into very fine fibers which float about in the air. 



If a small charged body such as a pith ball be suspended any- 

 where in the region between A and B y it will be found to be acted 

 upon by a force in the direction of the electric field, that is in the di- 

 rection of the lines of force at the point where the pith ball is placed. 



Need of large electromotive forces and good insulation. The 

 phenomena described above and, in fact, most of the phenomena 

 of electrostatics are easily perceptible only when the bodies are 

 charged by electromotive forces of many thousands of volts. 

 The most convenient means for producing these large electro- 

 motive forces is the Holtz or Wimshurst electrical machine. 

 When such a machine is used, however, the bodies A and B 

 must be well insulated, because such electrical machines cannot 

 supply charge at a rapid rate, that is such a machine can deliver 

 only a very small current. 



