164 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



leaf is connected to one terminal of a dynamo and the brass strip 

 to the other terminal, the gold leaf is attracted by the brass. It 

 is necessary in this arrangement to cover the face of the brass 

 strip with a layer of paper to avoid short-circuiting the dynamo 

 through the gold leaf. 



The outward pull of the electric field on the surface of a 

 charged body is very strikingly shown by pouring a viscous 

 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 

 surface of 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. 



Need of high electromotive force and of good insulation. The 

 phenomena described above, 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 method of producing these large electromotive 

 forces is by means of the Holtz or Wimshurst electrical machine, 

 and, when such a machine is used, the bodies A and B must be 

 well insulated, because such electrical machines cannot supply 

 charge at a rapid rate, that is, such machines can deliver only 

 very small currents. (See Arts. 106 and 108.) 



The electric spark. When the electromotive force acting to 

 charge two bodies A and B, Fig. 1020, is increased more and 

 more, a value is eventually reached which breaks down or ruptures 

 the dielectric and allows the charge on the bodies to pass in the 

 form of an electric spark. 



Mechanical analogue of electrically -charged bodies and of the 

 electric field. Imagine two cavities A and B, Fig. 102^, in an 

 extended elastic solid such as rubber or jelly. Imagine these 

 cavities to be filled with water and to be connected to a pump by 

 means of a pipe so that the pump may draw a certain amount of 

 water out of one cavity and force it into the other, thus causing 

 one cavity to contract and the other cavity to expand, and causing 

 the surrounding mass of rubber or jelly to be strained, the lines 



