CHAPTER I. 



PROPERTIES OF AN ELECTRIC CHARGE. 



FIRST I must lay a basis of pure theory : we must 

 consider the properties of the ancient and long known 

 phenomenon called an electrified body. 



Two substances placed in intimate contact and 

 separated are in general united more or less per- 

 manently by lines of force, the region between them 

 being in a state of tension along the lines and of 

 pressure at right angles. These lines have direction 

 and ' sense ' their two ends are not alike : they 

 begin at one body and end at another, they map out 

 a field of electrostatic force, and their terminations on 

 one or other of the bodies constitute what we call 

 an electric charge. Electric charges are of two kinds, 

 positive and negative, the former corresponding to 

 the beginning of the lines, the latter to their ends. 

 To one class of bodies, called insulators, the lines 

 appear rigidly attached : the charges cannot be dis- 

 placed nor transferred elsewhere without violence ; 

 whereas in another class they slip easily along, and 

 are transferred from one such conducting body to 

 another in contact with it, with great ease. 



A tension in the lines tends to bring the ends 

 together as near as possible, while laterally the lines 

 tend to drive each other apart : this image sufficing to 



L.E. 



