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SHEWING THE LINES OF EQUAL MAGNETIC DECLINATION AND THOSE OF EQUAL DIP & INTENSITY.

FOR THE YEAR, 1888.

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IN

ELECTRICITY & MAGNETISM

BY

SILVANUS P. THOMPSON, D.Sc., B.A., F.R.A.S.

PRINCIPAL OF AND PROFESSOR OF PHYSICS IN THE CITY AND GUILDS OF
LONDON TECHNICAL COLLEGE, FINSBURY;

LATE PROFESSOR OF EXPERIMENTAL PHYSICS IN

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Printed by R. & R. CLARK, November 1881.

Reprinted and corrected April, November 1882, 1883, 1884, 1885,
1886, 1887, 1889.

T43
1837

PREFACE.

THESE Lessons in Electricity and Magnetism are intended to afford to beginners a clear and accurate knowledge of the experiments upon which the Sciences of Electricity and Magnetism are based, and of the exact laws which have been thereby discovered. The difficulties which beginners find in studying many modern text-books arise partly from the very wide range of the subject, and partly from want of familiarity with the simple fundamental experiments. We have, at the outset, three distinct sets of phenomena to observe, viz.—those of Frictional Electricity, of Current Electricity, and of Magnetism; and yet it is impossible to study any one of these rightly without knowing something of them all. Accordingly, the first three Chapters of this work are devoted to a simple exposition of the prominent experimental facts of these three branches of the subject, reserving until the later Chapters the points of connection between them, and such parts of electrical theory as are admissible in a strictly elementary work. No knowledge of algebra beyond simple equations, or of geometry beyond the first book of Euclid, is assumed.

A series of Exercises and Problems has been added at the end of the Book in order that students, who

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 chm, and the ampère, as units of electromotive-force, resistance, and current, 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.

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, Thomson, 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. Though the Author does not think it well in an elementary text-book to emphasize particular theories on the nature of Electricity upon which the highest authorities are not yet agreed, he believes that it will add to a clear understanding of the matter if he states his own views on the subject.

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