plz help..12th grade physics problem?

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Another question:-
A wire of resistance R is stretched so that its radius decreases by factor n. Calculate its new resistance.
Why are the resistance coils made of Constantine and mangnin and double wound on non conducting frame????
Give 2 reasons why potentiometer is preferred is preferred over voltmeter for measuring E.M.F...
and please see and answer the other questioms too..
 
The DC resistance of a wire is inversely proportional to its cross sectional area. If the area doubles, it's resistance (per length) halves. So its new resistance will be based on the area that the new radius represents.
 
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