If a $0.1\%$ increase in length due to stretching, the percentage increase in its resistance will be ............ $\%$
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(a) $R \propto {l^2}$ $\Rightarrow $ $\frac{{\Delta R}}{R} = \frac{{2\Delta l}}{l}$
$\Rightarrow $ $\frac{{\Delta {R_0}}}{R}\% = 2 \times 0.1 = 0.2\% $
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