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March 7, 2008

3 light switches

There are three switches in a hallway. One switch controls a
light fixture in a room at the far end of the hall. The door
to the room is closed, and you can't see whether the light
is on or off. You need to find out which of the three switches
controls the light. How can you be certain of finding that out,
making just one trip to the room?


kARTHIK said...

I switch ON d first switch for sometime then, i OFF that one,then I switch ON the second. And i keep the other switch OFF.......
OK,then i go to the room havin d light.If it is ON then it belong to 2switch.if it is OFF with some heat when i touch, Then it belongs 1switch.else it belong TO 3switch.

Anonymous said...

You can do it for 4 bulbs (or more if you know how hot the glass gets over time). eg for 4 - turn Sw1 on for ten minutes; turn Sw1 off and Sw2 on for 2 minutes; turn off Sw2, turn on Sw3, and go to the room. If light is on it is Sw3. If it is off and HOT it is Sw1; off and WARM, Sw2, else Sw4.

Secret Squïrrel

Neeraj said...

heating effect of electricity in light emmission is the key factor.

Tulsi said...

if ur novice to heat energy then just switch ON all the 3, go open the door thn come back and switch off one by one and find it out whn it goes OFF

kishore said...

Switch on Switch-A.. wait for 5 min.. switch off the Switch1. Switch on Switch-B and go to the room. If the Bulb is on, then its Switch-B. If the Bulb is hot then its Switch-A else its Switch-C.

Anonymous said...

first you turn two light switches on. Then you wait ten-twenty minutes and then turn one off. you go into the room and look at the light bolb and feel the light bulb. if the light switch is on then its simply goes to the light switch thats on. if its hot but not on the its the light switch you just switched off. if its not on or not on that light swith goes to the one that was never on.