Gourang made an electric circuit with three bulbs `B_(1), B_(2)` and `B_(3)` a cell and two switches `S_(1)` and `S_(2)` as shown in figure. He notes his observation when

(a) `S_(1)` is closed and `S_(2)` is open,
(b) `S_(2)` is closed and `S_(1)` is open.
He also found that a bulb whose failure made the circuit 'open' irrespective of the status of `S_(1)` and `S_(2)` . What are his observation.

Explanation

(a) If `S_1` is closed and `S_2` is open, only `B_1 and B_2` glow (b) If `S_2` is closed and `S_1` is open, only `B_1 and B_3` glow. If `B_1` fails, the circuit becomes open, even if both `S_1` and `S_2` are closed.