Step 1: A cell balances at l₁ = 85.0 cm on open circuit. With R = 10.0 Ω connected across it, the balance shifts to l₂ = 80.0 cm. Find the internal resistance r. Step 2: Use r = R(l₁ − l₂)/l₂. Step 3: l₁ − l₂ = 85.0 − 80.0 = 5.0 cm. Step 4: r = 10.0 × 5.0/80.0 = 0.625 Ω. Step 5: The cell's EMF (standardized gradient 0.02 V/cm): E = 0.02 × 85.0 = 1.70 V; terminal voltage under load V = 0.02 × 80.0 = 1.60 V. Step 6: Check: V = ER/(R+r) = 1.70 × 10.0/10.625 = 1.60 V ✓. Result: r ≈ 0.63 Ω. A fresh dry cell is ~0.5–1 Ω; an old one rises to several ohms — which is why old cells can't drive heavy loads.