128] ELECTRICAL PHENOMENA. 247 



EXPERIMENT VIII. Suppose that while the pieces of electrified 

 glass are suspended as in Experiment I, we surround them with a 

 dielectric fluid insulator, such as turpentine, kerosene, or melted 

 paraffin. It will be found that, if the buoyancy of the liquid be 

 just counterbalanced by weights, the threads will now hang more 

 nearly vertically, showing that the repulsion is less. The energy 

 of the system is consequently less. We see then that the energy 

 of a system of electrified bodies depends not only on their charges 

 and positions, but on the nature of the dielectric medium in which 

 they are placed. The consideration of the part played by the 

 medium is now one of the principal parts of electrical theory. 



If any of the other experiments be repeated with the closed 

 vessel filled with any dielectric fluid, the results will be unchanged, 

 showing that the values of charges induced on a closed conductor 

 by charges within are unaffected by the dielectric. 



We will now briefly recapitulate the results of our experi- 

 ments. 



We may examine the nature and the magnitude of the charge 

 of any electrified body without altering it, by placing it within 

 an insulated hollow conductor without touching it, and examining 

 the charge induced on the outside of the latter. 



The amount of electricity on a body remains unchanged, unless 

 it be put in conducting communication with another body. 



When a body electrifies another by conduction, the quantity of 

 electricity on the two remains unchanged. 



When electricity is produced by friction (or otherwise, as we 

 shall find) equal quantities of positive and negative electricity are 

 produced. 



When electrification is caused by induction from a body 

 surrounded by a conductor, the amount of electricity on the inside 

 of the conductor is equal in quantity and opposite in sign to the 

 charge of the inducing body. 



There is no electricity on the inside surface of a closed hollow 

 conductor, charged but under the action of no internal bodies. 



The forces between charged bodies, and their electrical energy, 

 depend on the dielectric medium in which they are placed. The 

 charges induced on closed conductors by charges within do not. 



