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Chapter 27 - 27.21

21. The resistance of conductor A is given by

R A =

ρL

πr A

where r A is the radius of the conductor. If r o is the outside diameter of conductor B and r i is its inside

diameter, then its cross-sectional area is π ( r o −

r i ), and its resistance is

R B =

ρL

π ( r o −

r i ) .

The ratio is

R B = r o −

r i

= (1 . 0mm) 2

(0 . 50mm) 2

−

=3 . 0 .

r A

R A