. A text book of physics, for the use of students of science and engineering . , neither is it inthe geographical meridian. If the verti-cal plane AB (Fig. 730) be taken to repre-sent the geographical meridian, or planepassing through the point of observationand containing the axis of rotation ofthe earth, the magnetic meridian, or verticalplane containing the axis of a freelysuspended magnet, is inclined to AB, andmay be represented by the plane angle GCD between the geographicaland magnetic meridians is called themagnetic declination, or in nautical language,the magnetic variation,
. A text book of physics, for the use of students of science and engineering . , neither is it inthe geographical meridian. If the verti-cal plane AB (Fig. 730) be taken to repre-sent the geographical meridian, or planepassing through the point of observationand containing the axis of rotation ofthe earth, the magnetic meridian, or verticalplane containing the axis of a freelysuspended magnet, is inclined to AB, andmay be represented by the plane angle GCD between the geographicaland magnetic meridians is called themagnetic declination, or in nautical language,the magnetic variation, or the variation ofthe compass. The angle DCE between the resultantdirection of the earths magnetic field and the horizontal is calledthe magnetic dip ; it is the angle of dip of a magnetised needle whichis free to rotate in the plane of the magnetic meridian. The resultant magnetic intensity CE, represented by I, may beresolved into two components, H andV, one of which is horizontaland the other vertical. Since the triangles CDE and EFC have rightangles at D and F, we have. Fig. 730.—Magnetic declinationand dip. tan 6 VFT and I2 = H2 + V2. These quantities, the declination, dip, H, V and I are known astin magnetic elements at any point on the earths surface, and ifthe declination and any two of the other elements are known, thewhole of i hem may be calculated. The three elements usually measured at anyplace are the declina-tion, dip, and the horizontal component of the earths field, H. Themeasurement of H has already been described. Lxin MAGNETIC DECLINATION 795 Measurement of declination. To determine the declination atany place, the position of the geographical meridian must be found,and also that of the magnetic meridian. The geographical meridianis found by astronomical means, by observing the direction of the sunat a known time, its direction at this particular time being known interms of the longitude of the place, from astronomical tables. Todetermine the direction of
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics
