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| Local attraction ： ウィキペディア英語版 |
Local attraction
While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called as local attraction.〔surveying volume 1 by Dr B.c Punmia and Ashok Kumar Jain. Chapter Compass Surveying〕 The external forces are produced by sources of local attraction which may be current carrying wire (magnetic materials) or metal objects.〔surveying by prof. Sk Duggal available on internet from Google books〕 The term is also used to denote amount of deviation of the needle from its normal position. It mostly causes errors in observations while surveying and thus suitable methods are employed to neglect these errors.〔Surveying by prof. Cl Kocher ,Principal of Mehar chand Polytechnic College, Jalandhar City, Punjab/Compass Surveying〕
==Sources==
The sources of local attraction may be natural or artificial. Natural sources include Iron ores or magnetic rocks while as artificial sources consist of steel structures, iron pipes, current carrying conductors. The iron made surveying instruments such as metric chains, ranging rods and arrows should also kept at a safe distance apart from compass.〔surveying by Prof. Cl Kocher〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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