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JOHN TROWBRIDGE, S. D.
RUMFORD PROFESSOR AND 'LECTURER ON THE APPLICATIONS OF SCIENCE
TO THE USEFUL ARTS, HARVARD UNIVERSITY, AND DIRECTOR

OF THE JEFFERSON PHYSICAL LABORATORY

ILLUSTRATIL

NEW YORK
D. APPLETON AND COMPANY

1903

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PREFACE.

I am often asked the question, “What is electricity ?” and I have endeavoured in this book to give in a popular manner the present views of scientific men in regard to this question. According to modern ideas, the continuance of all life on the earth is due to the electrical energy which we receive from the sun; and physics, in general, can be defined as that subject which treats of the transformations of energy. I have therefore presented the varied phenomena of electricity in such a manner that the reader can perceive the physicist's reasons for supposing that all space is filled with a medium which transmits electro-magnetic waves to us from

the sun.

In Tyndall's Heat as a Mode of Motion, Tait's Recent Advances in Physical Science, and Stewart's Conservation of Energy, the relations between work done and heat produced have been treated in a popular manner; but I am not acquainted with any treatise in which Maxwell's great generalization, entitled the ElectroMagnetic Theory of Light, has been made the basis of a popular treatment. The wide-embracing nature of this theory can be seen when we realize that, according to it, all phenomena of light, heat as well as those of electricity, are manifestations of electrical energy.

I have used in this treatise various popular lectures which I have delivered, and certain articles which I have published in the Chautauquan, the Popular Science Monthly, the American Journal of Science, and the London Philosophical Magazine.

I realize fully the difficulty of stating accurately in a popular exposition what is more definitely expressed in mathematical language. If I have succeeded in giving the general reader an idea of the present direction of investigation in the science of electricity I shall consider myself fortunate.

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