File:An elementary book on electricity and magnetism and their applications (1919) (14803698263).jpg

Identifier: elementarybookon00jack (find matches)
Title: An elementary book on electricity and magnetism and their applications
Year: 1919 (1910s)
Authors: Jackson, Dugald C. (Dugald Caleb), 1865-1951 Jackson, John Price, b. 1868 Black, Newton Henry, b. 1874
Subjects: Electricity Magnetism
Publisher: New York, The Macmillian Co
Contributing Library: Internet Archive
Digitizing Sponsor: Internet Archive

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e earths magnetic fieldand of the variation and dip at anylocality show that these do not remainfixed but change from time to time. It has been found thatthere is a secular change which occurs once a century, anotherwhich is annual, and that there are still others which occurdaily. Besides all these, there are erratic disturbances whichoccur simultaneously over the whole earth and are called mag-netic storms. Thus it is necessary to erect at certain observ-atories recording instruments or magneto graphs, which givea permanent record of the slight changes in the terrestrialmagnetic field. It is of such great importance to determine the exactdirection of the earths magnetic lines of force or magneticmeridians, not only on land but particularly on the oceans,that the Carnegie Institute has had a special ship built,the Carnegie, almost entirely from nonmagnetic materials.This vessel with its delicate instruments is making an accu- Fig. 80. — Compass needleto show magnetic dip. PLATE VIL
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William Thomson, Lord Kelvin (1824-1907). A British physicist and electrical engineer ; horn in Belfast, Ireland ; for morethan fifty years professor of physics in Glasgow University, Scotland; in-vented the mirror galvanometer used in cable signaling; developed an im-proved form of mariners compass. His work in thermodynamics is of thegreatest value. He was one of the first to appreciate the principle of Con-servation of Energy. NATURE AND PROPERTIES OF MAGNETISM 133 rate magnetic survey of the important portions of the earthssurface. 100. A ships compass. The form of ships compass whichis now quite generally used was introduced by Lord Kelvin(Plate VII, opposite p. 133). It has a ten-inch card consisting ofa thin sheet of aluminum or paper on which a scale is pasted, ordrawn, and varnished. The middle portion is cut out for the sake

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