a
\(\frac{\mathrm{R}_{1}}{\mathrm{R}_{2}}=\frac{\ell_{1}}{\ell_{2}}= \frac{\mathrm{I}_{2}}{\mathrm{I}_{1}}=\frac{\theta_{1}}{\theta_{2}} \Rightarrow \theta_{1} \mathrm{I}_{1}=\theta_{2} \mathrm{I}_{2}\)

\(\left.\begin{array}{l}{\mathrm{B}_{1}=\frac{\mu_{0}}{4 \pi} \frac{\mathrm{I}_{1} \theta_{1}}{\mathrm{r}}} \\ {\mathrm{B}_{2}=\frac{\mu_{0}}{4 \pi} \frac{\mathrm{I}_{1} \theta_{1}}{\mathrm{r}}}\end{array}\right\} \Rightarrow \mathrm{B}_{1}=\mathrm{B}_{2}\)