The Continuous Variation Plate 



THUMB SCREW 





Fiff. 48 Continuous Variation Plate, Showing Inclined Reading 

 face. Vertical Graduations and Vernier Scale. Illustration shows circle 

 set for a variation of 7 K. 



The magnetic declination at any place is the angle contained 

 between two vertical planes, one being the true meridian and the 

 other the plane in which the axis of a freely suspended needle will 

 lie, as an algebraic result of the various periodic and irregular 

 influences that control the direction of the needle. Among these 

 may be briefly mentioned: 



Terrestrial Magnetism, which is the principal directive force 

 induced by the currents that pass between the earth's magnetic poles. 



Secular Variation, which shows charted oscillations as great 

 as 24 in one direction spread, however, over a cycle of years that 

 has not yet been closed since magnetic observations have been a 

 matter of record. 



Annual Deviation, which changes mainly with the seasons, 

 being more conspicuous in summer than in winter, varying roughly 

 between 5' and 15' per year. 



Diurnal Variation, being a systematic angular movement of 

 from 5' to 10' covering a solar day, doubtless due to the attraction 

 of the sun. 



Local Attraction, which is very noticeable in the vicinity of 

 slate, pyrrhotite, magnetite deposits, or industrial or electric plants. 

 It not only affects the horizontal position of the needle but frequently 

 interferes with the vertical balance. 



Lunar Inequalities, Solar Phenomena and Magnetic 

 Storms, which often exert sudden deflections that are beyond the 

 of prediction. Magnetic disturbances accompanied by 



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