Chapter 21 - Electric Current and Direct-Current Circuits - Problems and Conceptual Exercises - Page 755: 14

$5.24\times 10^{-4}m$

We know that $R=\frac{\rho L}{A}$ $R=\frac{\rho L}{\frac{\pi d^2}{4}}$ We plug in the known values to obtain: $0.07=\frac{5.67\times 10^{-8}(0.27)}{\frac{\pi d^2}{4}}$ $0.07=\frac{1.926\times 10^{-8}}{d^2}$ $\implies d^2=\frac{1.926\times 10^{-8}}{0.07}$ $\implies d^2= 27.5\times 10^{-8}$ $\implies d=5.24\times 10^{-4}m$