MAGNETISM AND ELECTROMAGNETISM

Chapter 14 MAGNETISM AND ELECTROMAGNETISM

Lessons In Electric Circuits Volume I – DC Book
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Lessons In Electric Circuits Volume I – DC Book

  • Chapter 14MAGNETISM ANDELECTROMAGNETISMContents 471,14.1 471,Permanent 471,magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 475,14.2 475,Electromagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 477,14.3 477,Magnetic 477,units 477,of 477,measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 467 480,14.4 480,Permeability 480,and 480,saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 485,14.5 485,Electromagnetic 485,induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475 487,14.6 487,Mutual 487,inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477 490,14.7 490,Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48014.1Permanent magnetsCenturies ago, it was discovered that certain types of mineral rock possessed unusual prop-erties of attraction to the metal iron. One particular mineral, called lodestone, or magnetite,is found mentioned in very old historical records (about 2500 years ago in Europe, and muchearlier in the Far East) as a subject of curiosity. Later, it was employed in the aid of navigation,as it was found that a piece of this unusual rock would tend to orient itself in a north-southdirection if left free to rotate (suspended on a string or on a float in water). A scientific studyundertaken in 1269 by Peter Peregrinus revealed that steel could be similarly ”charged” withthis unusual property after being rubbed against one of the ”poles” of a piece of lodestone.Unlike electric charges (such as those observed when amber is rubbed against cloth), mag-netic objects possessed two poles of opposite effect, denoted ”north” and ”south” after theirself-orientation to the earth. As Peregrinus found, it was impossible to isolate one of thesepoles by itself by cutting a piece of lodestone in half: each resulting piece possessed its ownpair of poles:461