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.