Worked example: With P = 100 Ω, Q = 10 Ω plugs in, balance is found at R = 437 Ω. Find S. Then explain how you would measure a 0.2 Ω resistor. Step 1: S = R·Q/P = 437 × 10/100 = 43.7 Ω. Step 2: For 0.2 Ω, choose P:Q = 1000:10 (ratio 100:1), so S = R/100 — dial R near 20 Ω for balance at S = 0.2 Ω. Step 3: Why this ratio? It puts the dial reading in a comfortable mid-range where plug resistances (milliohms) are negligible fractions. Step 4: Key discipline — battery key down first, tap the galvanometer key briefly, adjust R, repeat; never hold the galvanometer key down while far from balance.