PREFACE AND INTRODUCTION. 



"Alles Vergangliche ist nur ein Gleichniss." 

 (Intelligibility is only a likeness. ) 



The elementary theory of electricity and magnetism is essen- 

 tially an extension of the science of mechanics,* and the purpose 

 of this book is to develop the science of electricity and magnetism 

 from this point of view. 



The study of elementary physics, in one of its important phases 

 is imaginative like the study of geometry, its purpose is to ration- 

 alize our experience of physical conditions and things, and the 

 building up of the rational structure of physics should be the 

 chief function of a text-book for students. This text has been 

 prepared in accordance with this idea. 



The attempt has been made throughout to bring simple prac- 

 tical applications into the mind of the student. It would perhaps 

 be ridiculous in a descriptive treatise on physics for college men 

 to consider in detail those things which are universally and per- 

 fectly known, but it is precisely such things that should be referred 

 to in a rational treatise. If one is to rationalize, one must ration- 

 alize about something. It is a mistake, however, to shape science 

 instruction prematurely to practical (economic) ends, but such 

 "practical " instruction is a very different thing from the rational 

 study of the things of everyday life. Elementary science instruc- 

 tion must be made to touch upon the things of everyday life if it is 

 to be effective. In no other way can what is best in science be 

 realized anew in each succeeding generation of men. 



*See Art. 125 on the distinction between the mechanical theory and the atomic 

 theory. 



Special attention is called to Art. 62 on the mechanical aspect of Lenz's Law ; to 

 Chapter VI on Inductance ; to Art. 89 on the mechanical analogue of the condenser ; 

 to Arts. 106, 107 and 108 on the mechanical analogies of electric doubling ; and to 

 Chapter IX on the mechanical conceptions of the electromagnetic field and of electro- 

 magnetic waves. 



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