244 ELECTROSTATICS. [PT. II. CH. VI. 



The two pieces of resin in like manner repel each other. Each 

 piece of glass attracts each piece of resin. All of these phenomena, 

 each of which indicates the acquisition of a positive amount of 

 potential energy, are known as Electrical phenomena, and the 

 bodies exhibiting them are said to be electrified, or charged with 

 Electricity. 



The properties of the two pieces of glass are similar, but 

 opposite to those of the resin. What the glass attracts the resin 

 repels, and vice versa. Bodies repelled by the glass and attracted 

 by the resin are said to be vitreously, those attracted by the glass 

 and repelled by the resin, resinomly electrified. By general 

 convention we say positive, instead of vitreous, negative for 

 resinous. 



EXPERIMENT II. Let a hollow metal vessel be hung up by 

 silk threads, and let a lid completely closing it be also so hung, so 

 that it may be removed and replaced without touching it. Then 

 if the electrified glass be hung inside the vessel without touching 

 it, and the lid placed on, the outside of the vessel will be found 

 vitreously electrified, and the manner of the electrification will be 

 exactly the same in whatever part of the interior the glass may be. 

 That is to say, if we place successively at different points of the 

 external space the same small electrified body, it will be acted 

 upon at each point by a certain force. The direction and magni- 

 tude of this force determine a vector called the strength of the 

 electrical field of force. The field may be geometrically repre- 

 sented by lines of force in the usual manner. The electric field is 

 the tangible evidence of the electrification, and the measurement 

 of a force is the means of its measurement. We may therefore 

 describe the above experiment by saying that the field external to 

 the closed metal vessel is independent of the position of the charged 

 body within. If the glass be removed without touching the vessel, 

 the electrification of the glass will be unchanged, and that of the 

 vessel will have disappeared. If resin be substituted for glass the 

 outside of the vessel will be negatively electrified. Such electri- 

 fication, which depends on the proximity of electrified bodies, is 

 called electrification by influence, or induction. In this manner a 

 body may acquire energy without contact with other bodies, and 

 it is natural to suppose that the energy has passed through the 

 intervening medium from the electrified body. Such a medium, 



