VI PEE FACE. 



book was nearly ready for the press, while extremely modern as well 

 as clear, is addressed to a somewhat different class of students 

 from that contemplated in writing the present book. 



The theoretical writings of Hertz, Heaviside, Cohn and others 

 have resulted in the systematization of Maxwell's theory and have 

 made possible improvements in the mode of its presentation and 

 nomenclature not contemplated by him. The extremely important 

 and original contributions of Mr Oliver Heaviside are unfortunately 

 but little adapted to the use of the student on account of their 

 very voluminous character as a whole, as well as of an extreme 

 conciseness of expression in individual parts. The few brilliant 

 chapters on theoretical matters left by Hertz are hardly by way 

 of exposition, but rather of a summing up of the conclusions of 

 the theory. 



It has been my aim in the preparation of this volume to 

 present to the student the results of the theory as it stands to- 

 day after the labors of Faraday, Maxwell, Helmholtz, Hertz and 

 Heaviside. Here it may be convenient to state what I consider 

 to be the essentials of Maxwell's theory as distinguished from 

 the old theories. To this question may very well be made the 

 answer of Hertz : " Maxwell's theory is Maxwell's system of equa- 

 tions." But to specify more fully the points of difference, they are 

 in the opinion of the writer : 



1. The localization of the energy in the medium. 



2. The magnetic action of displacement currents. 



While starting from the standpoint of Energy, I have not 

 thought it advisable to abolish the usual terms repugnant to so 

 many writers, who assuming the attitude of Maxwellians par 

 excellence, deny the existence of Electricity. Maxwell himself 

 was not one of these. Feeling that the consideration of the 

 Newtonian Potential Function is indispensable, not only for the 

 old theory of action at a distance, but for the modern theory, and 

 in addition that it introduces the student to many of the methods 

 that he will need in various branches of mathematical physics, I 



