switches

graeme · 2003-10-16, 10:05

So for some strange reason you are shown a room with three unlit desktop lamps and then moved to another room down the corridor where you are shown three switches which control the lights.
From this room you can't see whether the lights in the first room are off or on.
You are allowed to fiddle with the switches for as long as you want, then you are taken back to the first room (where the lamps are) and you have to say which switch controls which light.
How can you do this?

Graeme

Wacky · 2003-10-16, 10:36

I "fiddle" with the switches by unwiring them. Then, I can truthfully say that none of the switches control any of the lights.

Or do you intend to restrict the situation to the case where each switch controls exactly one light and each light is controlled by only one switch?

eepiccolo · 2003-10-16, 13:34

You ask the person who took you between rooms to watch the lights as you switch the switches, and report which ones go on and off.

graeme · 2003-10-16, 15:58

OK. I say that Wacky didn't get this one since he's still not able to say which switch controls which lamp (instead, as he says, he can say which switches don't control the lamps). Actually in his second paragraph he just stops short of saying that he could wire them all together in one circuit, so I'll give him half a point.

I grudgingly concede eepiccolo's answer, alas, since he used all of the information in an usual manner.

Let me try to refine the puzzle (doomed to failure now, especially since I've provoked Wacky, but here goes anyway):

So as Wacky said in the end part of his message, each switch controls one and only one of the lamps and your only allowed interaction with the switch is to turn it off and on.
No-one else is around to help you, the only way of checking the state of the lamps in the first room is by going there, which you can only do once.

Graeme

Wacky · 2003-10-16, 16:38

Ok, although I protest that you gave me only partial credit.

Since no one is around to help me, I'll dig in the file cabinet and find the Electrical Diagram for the building. (It would have been easier to just ask the Building Super for it, but you don't allow that).

If that doesn't make it clear, I'll follow your hint and turn on switch #1 before returning to the room with the lamps. There I should see one light on. I'll plug the other two into the same outlet and now announce that #1 controls all of them.

If you deny all of our "re-engineering" answers, I don't think that it can be done. There are 6 possible ways that the switches can be connected to the lamps and at most 3 possible outcomes displayed by the lamps.

I await your solution.

roy1942 · 2003-10-16, 17:58

I've PM'd the proposer with an answer that I think works, and a (trickier) answer to the same problem but with four switches and lamps. Are we ready for serious answers in public yet?

only_human · 2003-10-16, 18:33

I leave turn on switch 1 immediately, turn on switch 2 after 1 minute, turn on switch 2 after 2 minutes. One minute later, I turn off all three switches and run to the other room using fingers, wrists and lips extremely carefully to guess the temperature of the three bulbs.

smh · 2003-10-16, 18:48

I would do it the other way around. Switch on two lights, wait a while before switching off one of them, and go to the other room.

1 light should still be on, one should be cold and one should be warm.

Fusion_power · 2003-10-17, 00:01

I protest!

You stipulated that they were "desktop" lamps. So are they fluorescent or incandescent? If they are fluorescent, you will have a much more difficult task to guage the heat for the two you turn on.

Fusion

GP2 · 2003-10-17, 02:36

Quote:

Originally posted by Fusion_power
If they are fluorescent, you will have a much more difficult task to guage the heat for the two you turn on.

Fluorescent lights continue to fluoresce weakly for a while even after they are turned off.

However the bulb could conceivably be some newfangled halogen technology or whatnot, so you can't really rely on the physical properties of the light. You can't even be sure that all three lamps use the same kind of bulbs.

You are allowed to fiddle with the switches "as long as you want". You could therefore do so for several years or decades, or well past the MTBF of any known lightbulb technology. Cycling the power on and off continuously will probably stress any bulb enough to ensure its early demise.

Then an obvious solution presents itself: one lamp on, one lamp off, the third lamp visibly burned out -- always assuming that you can tell by visual inspection whether a bulb has failed, which is usually true for incandescent but perhaps not for fluorescent bulbs.

I suspect the solution must make some assumption about the physical properties of the bulbs: residual heat, residual fluorescence, or visually obvious evidence of being burnt out.

I also suspect that "desktop lamp" is supposed to be a clue that the bulbs must be incandescent, although with today's energy-saving compact fluorescent bulbs that's no longer a given.

dweick · 2003-10-17, 03:28

My solution also relies on physical properties of the bulbs and assumes very reliable switches. I would switch one switch on and off several million times. I would then turn that switch off and turn one other switch on.

I would then go to the room, the light that is on is controlled by the switch that I just turned on. I would then either examine the other two lamps to see which one had a burned out bulb or plug them one by one into the powered socket to see which one was still working.

Might not work with LED based lamps but then again checking their temperature probably wouldn't work either.

The original problem did say I could fiddle with the switches as long as I wanted!

2003-10-16, 16:38	#5
Wacky Jun 2003 The Texas Hill Country 3²×11² Posts	Ok, although I protest that you gave me only partial credit. Since no one is around to help me, I'll dig in the file cabinet and find the Electrical Diagram for the building. (It would have been easier to just ask the Building Super for it, but you don't allow that). If that doesn't make it clear, I'll follow your hint and turn on switch #1 before returning to the room with the lamps. There I should see one light on. I'll plug the other two into the same outlet and now announce that #1 controls all of them. If you deny all of our "re-engineering" answers, I don't think that it can be done. There are 6 possible ways that the switches can be connected to the lamps and at most 3 possible outcomes displayed by the lamps. I await your solution. Last fiddled with by Wacky on 2003-10-16 at 16:40

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2003-10-16, 10:05	#1
graeme Jul 2003 51₈ Posts	switches So for some strange reason you are shown a room with three unlit desktop lamps and then moved to another room down the corridor where you are shown three switches which control the lights. From this room you can't see whether the lights in the first room are off or on. You are allowed to fiddle with the switches for as long as you want, then you are taken back to the first room (where the lamps are) and you have to say which switch controls which light. How can you do this? Graeme

2003-10-16, 10:36	#2
Wacky Jun 2003 The Texas Hill Country 2101₈ Posts	I "fiddle" with the switches by unwiring them. Then, I can truthfully say that none of the switches control any of the lights. Or do you intend to restrict the situation to the case where each switch controls exactly one light and each light is controlled by only one switch?

2003-10-16, 13:34	#3
eepiccolo Dec 2002 Frederick County, MD 2·5·37 Posts	You ask the person who took you between rooms to watch the lights as you switch the switches, and report which ones go on and off.

2003-10-16, 15:58	#4
graeme Jul 2003 41 Posts	OK. I say that Wacky didn't get this one since he's still not able to say which switch controls which lamp (instead, as he says, he can say which switches don't control the lamps). Actually in his second paragraph he just stops short of saying that he could wire them all together in one circuit, so I'll give him half a point. I grudgingly concede eepiccolo's answer, alas, since he used all of the information in an usual manner. Let me try to refine the puzzle (doomed to failure now, especially since I've provoked Wacky, but here goes anyway): So as Wacky said in the end part of his message, each switch controls one and only one of the lamps and your only allowed interaction with the switch is to turn it off and on. No-one else is around to help you, the only way of checking the state of the lamps in the first room is by going there, which you can only do once. Graeme

2003-10-16, 17:58	#6
roy1942 Aug 2002 101111₂ Posts	I've PM'd the proposer with an answer that I think works, and a (trickier) answer to the same problem but with four switches and lamps. Are we ready for serious answers in public yet?

2003-10-16, 18:33	#7
only_human "Gang aft agley" Sep 2002 2×1,877 Posts	I leave turn on switch 1 immediately, turn on switch 2 after 1 minute, turn on switch 2 after 2 minutes. One minute later, I turn off all three switches and run to the other room using fingers, wrists and lips extremely carefully to guess the temperature of the three bulbs.

2003-10-16, 18:48	#8
smh "Sander" Oct 2002 52.345322,5.52471 29·41 Posts	I would do it the other way around. Switch on two lights, wait a while before switching off one of them, and go to the other room. 1 light should still be on, one should be cold and one should be warm.

2003-10-17, 00:01	#9
Fusion_power Aug 2003 Snicker, AL 7×137 Posts	I protest! You stipulated that they were "desktop" lamps. So are they fluorescent or incandescent? If they are fluorescent, you will have a much more difficult task to guage the heat for the two you turn on. Fusion

2003-10-17, 03:28	#11
dweick Apr 2003 12₈ Posts	My solution also relies on physical properties of the bulbs and assumes very reliable switches. I would switch one switch on and off several million times. I would then turn that switch off and turn one other switch on. I would then go to the room, the light that is on is controlled by the switch that I just turned on. I would then either examine the other two lamps to see which one had a burned out bulb or plug them one by one into the powered socket to see which one was still working. Might not work with LED based lamps but then again checking their temperature probably wouldn't work either. The original problem did say I could fiddle with the switches as long as I wanted!