In a Wheatstone bridge, arms have resistances P, Q, R, S in cyclic order with a galvanometer between the P-Q and R-S junctions. The bridge is balanced when

1. In a Wheatstone bridge the galvanometer reads zero at balance. 2. The condition for zero galvanometer current is P / Q = R / S. 3. Equivalently P S = Q R (cross-product form). 4. Neither the sum condition nor P × Q = R × S is the balance criterion. _Source: syllabus Physics, Ch 3 "Current Electricity", §3 Wheatstone bridge_