Wheatstone bridge is an arrangement of four resistances P, Q, R, and S as shown in the below figure: Any two diagonally opposite terminals (A & C) are connected to a source of emf E and the other two diagonally opposite terminals (D & B) are connected to a galvanometer.
The current distribution through different branches is shown in the above figure. In normal conditions, a current ig passes through the galvanometer and consequently, the needle of the galvanometer gets deflected. Under this condition, the bridge is said to be unbalanced.
If the values of the resistances P, Q, R, and S of the four arms are so adjusted that no current passes through the galvanometer (ig = 0), the bridge is said to be a balanced condition.