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Light bulbs

  1. Aug 2, 2008 #1
    You're in a room with three switches that open three lightbulbs in a separate room. You don't know which switch turns on which lightbulb. How do you find out which switch goes to which lightbulb by entering the other room only once? You don't have any help, you can't see the other room while you are in the room with the other switches or vice versa, you can't make any device that extends your reach into the other room, etc.
  2. jcsd
  3. Aug 2, 2008 #2

    James R

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    If the switches are numbered 1, 2 and 3, do this:

    Start with all switches off.
    Turn on switches 1 and 2.
    Wait 5 minutes.
    Turn off switch 2.
    Go into the other room.

    The light that is on corresponds to switch 1.
    The light that is off and warm corresponds to switch 2.
    The light that is off and cold corresponds to switch 3.
  4. Aug 2, 2008 #3


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    To give other people a chance to think about the problem, check out the "eye" icon in the editor menu:

    It helps the people who do not want to see the answer skip your posted solution.
  5. Sep 10, 2008 #4

    James R

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    Ah. Ok. I'll use that in future.
  6. Sep 11, 2008 #5
    This is a test of the eye function - DO NOT READ under penalty of law.
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