

| S.No. | $R$ | $l$ | $100-l$ | $S = \left( {\frac{{100 - l}}{l}} \right)R$ |
| $1$ | $20\,\Omega $ | $43$ | $57$ | $26.51\,\Omega $ |
| $2$ | $30\,\Omega $ | $51$ | $49$ | $28.82\,\Omega $ |
| $3$ | $40\,\Omega $ | $59$ | $41$ | $27.80\,\Omega $ |
| $4$ | $60\,\Omega $ | $70$ | $30$ | $25.71\,\Omega $ |


Reason : Kirchoff 's loop rule follows from conservation of momentum.
(Given resistivities of iron and copper-nickel alloy wire are $12 \;\mu \Omega {cm}$ and $51\; \mu \Omega {cm}$ respectively) (in ${m}$)