Worked example: In a meter bridge, the null point is found at l = 40 cm from the left (unknown S on the left, R = 10 Ω on the right). Find S. If the null shifts to 60 cm, what does that tell you? Step 1: S/R = l/(100 − l) — careful with sides: resistance ratio equals length ratio on the same sides, so S/10 = 40/60. Step 2: S = 10 × (40/60) = 6.67 Ω. Step 3: If the null shifts to 60 cm: S = 10 × 60/40 = 15 Ω. The null moves toward the larger resistance — a longer wire length balances a larger resistance, which is the intuitive rule for placing R. Step 4: Accuracy note — 40 cm is within the preferred 40–60 cm window, so this reading is near-optimal; at l = 95 cm, a 0.5 cm misreading would change the ratio by ~10%, versus ~2% here.