ICSE Class 10 Answered

Let a switch S₁ be fitted at the bottom and a switch S₂ at the top of the staircase. Fig. (a) shows the off position of the bulb.

The bulb can now be switched on independently by either the switch S₁ or the switch S₂. If the switch S₁ is operated, the connection 'ab' is changed to 'bc', which completes the circuit and the bulb lights up [Fig. (b)].

Similarly, on operating the switch S₂, the connection 'bc' changes to 'ba', which again completes the circuit [Fig. (c)].

Similarly if the bulb is in on position as shown in Fig. (b) or (c), one can switch off the bulb either from the switch S₁ or the switch S₂.