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LINES OF EQUAL MAGNETIC DECLINATION AND
THOSE OF EQUAL MAGNETIC DIP.

IN

ELECTRICITY & MAGNETISM

BY

SILVANUS P. THOMPSON,

D.So., B.A., F.R.S., 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

UNIVERSITY OOLLEGE, BRISTOL

NEW EDITION, REVISED THROUGHOUT

WITH ADDITIONS

New York
THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., LTD.

All rights reserved

COPYRIGHT, 1894,
BY MACMILLAN AND CO.

Set up and electrotyped December, 1894. Reprinted
June, 1895; March, December, 1896; October, 1897: August
1898; August, 1899; May, 1900: January, September, 1901 :
March, December, 1902; October, 1903; April, 1904; April,
December, 1905 ; July, 1906 ; Sept mber, 1907 ; April, 1908:
August, 1909 ; July, 1910; January, 1911; September, 1912.

PREFACE

THESE Elementary Lessons have now been largely rewritten. The considerable changes made have been necessitated not only by the progress of the science but by the piracy, covert as well as open, to which since its appearance in 1881 the book has been subjected.

In the thirteen years which have elapsed much addition has been made to our knowledge, and many points then in controversy have been settled. The system of electric units, elaborated first by the British Association and subsequently in several International Congresses, is now legalized in the chief civilized countries. New mag

- in England by Thorpe and Rücker, in the United States under Mendenhall — have enabled new magnetic charts to be prepared for the epoch 1900 A.D. The researches of Ewing, Hopkinson, and others on the magnetic properties of iron, and the general recognition of the principle of the magnetic circuit, have advanced the science of magnetism, to which also Ewing's molecular theory has given an added interest. The properties of alternate currents, of which in 1881 little was known, have been forced into study by the extension of their industrial uses in telephony and in electric lighting.

netic surveys

287991

Entirely new is the use of polyphase alternate currents and rotatory magnetic fields for the electric transmission of power. Transformers have come into extensive employment for the distribution at low-pressure of electric energy which has been transmitted from a generating station at high-pressure. Accumulators for the storage of electric energy have become of great commercial importance. Electric lamps, large and small, illuminate in millions our cities, towns, villages and ships. Electric currents for lighting and power are now supplied publicly on a very large scale from central stations operated by steam or water power. Supply-meters are in regular use, and measuring instruments of many forms have come into the market.

Along with these advances in practice there has been a no less striking progress in theory. The ideas of Faraday, as enlarged and developed by Clerk Maxwell, were in 1881 only beginning to be understood and appreciated outside a narrow circle. In 1894, thanks largely to the labours of Heaviside, Hertz, Lodge, Poynting, Fitzgerald, Boltzmann, Poincaré, and others, they are everywhere accepted. In 1881 Maxwell's electromagnetic theory of light - a conception not less far-reaching than the theory of the conservation of energy - was deemed of doubtful probability: it was not yet accepted by such great masters as Lord Kelvin or Von Helmholtz. Though adopted by the younger generation of British physicists, it needed the experimental researches of Hertz and of Lodge upon the propagation of electric waves to demonstrate its truth to their brethren in Germany, France, and Americ Even now, after the most convincing experimental verifications

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