In this treatise I have tried to present in systematic and definite form a simple, rigorous, and thoroughly modem introduction to the fundamental principles of electromagnetic theory, together with some of the simpler of their more interesting and important non-technical applications. The work makes no pretense to completeness, but is written for the serious student of physics, who will make liberal use of more detailed treatises, of hand-books, and of journals, as occasion demands. I am of course indebted to many books and memoirs. My obligations are especially great, as the most cursory examination of the book will show, to the works of Maxwell, Heaviside, and Poynting. I am also much indebted to Professor A. G. Webster for the use of a number of excellent diagrams from his treatise on electrical theory.