.ELEMENTARY LESSONS ON [CHAP. iv. 



CHAPTER IV, 



ELECTROSTATICS. 



LESSON XX. Theory of Potential. 



233. By the Lessons in Chapter I. the student will 

 have obtained some elementary notions upon the exist- 

 ence and measurement of definite quantities of electricity. 

 In the present Lesson, which is both one of the hardest 

 and one of the most important to the beginner, and 

 which he must therefore study the more carefully, the 

 laws which concern the magnitude of electrical quantities 

 and their measurement are more fully explained. In no 

 branch of knowledge is it more true than in electricity, 

 that " science is measurement." That part of the science 

 of electricity which deals with the measurement of 

 charges of electricity is called Electrostatics. We 

 shall begin by discussing first the simple laws of electric 

 force, which were brought to light in Chapter I. by 

 simple experimental means. 



234. First Law of Electrostatics. Electric 

 charges of similar sign repel one another^ but electric 

 charges of opposite signs attract one another. The funda- 

 mental facts expressed in this Law were fully explained 

 in Lesson I. Though familiar to the student, and 

 apparently simple, these facts require for their complete 

 explanation the aid of advanced mathematical analysis. 

 They will here be treated as simple facts of observation. 



