A metal wire of specific resistance $64 \times {10^{ - 6}}\,ohm - cm$ and length $198\, cm$ has a resistance of $7\, ohm$, the radius of the wire will be ............. $cm$
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(c) $R = \rho \frac{l}{A} \Rightarrow 7 = \frac{{64 \times {{10}^{ - 6}} \times 198}}{{\frac{{22}}{7} \times {r^2}}}$ $\Rightarrow $ $r = 0.024\,cm$
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