PREFACE. 



so desire, may test their power of applying thought to 

 what they read, and of ascertaining, by answering the 

 questions or working the problems, how far they have 

 digested what they have read and made it their own. 



Wherever it has been necessary to state electrical 

 quantities numerically, the practical system of electrical 

 units (employing the volt^ the ohm^ and the weber^ as 

 units of electromotive -force, resistance, and quantity, 

 respectively) has been resorted to in preference to any 

 other system. The Author has adopted this course 

 purposely, because he has found by experience that 

 these units gradually acquire, in the minds of students 

 of electricity, a concreteness and reality not possessed 

 by any mere abstract units, and because it is hoped 

 that the Lessons will be thereby rendered more useful 

 to young telegraphists to whom these units are already 

 familiar, and who may desire to learn something of the 

 Science of Electricity beyond the narrow limits of their 

 own practical work. A few historical references have 

 been added, without, it is hoped, encumbering the 

 Lessons. 



Students should remember that this little work is but 

 the introduction to a very widely -extended science, and 

 those who desire not to stop short at the first step should 

 consult the larger treatises of Faraday, Maxwell, Thom- 

 son, Wiedemann, and Mascart, as well as the more 

 special works which deal with the various Technical 

 Applications of the Science of Electricity to the Arts 

 and Manufactures. To these larger treatises the Author 

 is indebted on many points, though he alone is respon- 

 sible for any error which may have slipped into these 

 Lessons. 



