Yeah, you’re right. I mention later in the thread that I’d misconceived the requirements (by assuming that he’d start from the top, since in a real world situation the building owner would have shown him the fans) and noted it above. I’ma feel dumb now… @thedude541 has the better answer - one fan on, one fan off, one fan with switch leg pinned to ground. Five minutes and one hour of work counting one hour of climbing stairs.
Puzzles like this are always ambiguous because their primary purpose is to allow the person offering the puzzle to feel smug. I mean, was the puzzle supposed to test real world electrical knowledge like the answer @thedude541 gives? Or is it supposed to be a logic puzzle?
In my mind, if an answer is given to a puzzle and the puzzle-giver has to add a stipulation to invalidate that answer, they invalidate their puzzle. So I’ll just use my laser mircophone to listen for which fan is on from the ground, and make zero trips up.
There are three words in the English language ending in -gry, you know.
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He’s hired to fix the wiring. So he disconnects the existing wires and runs new wires from the switches to the fans up the stairwell. He documents which fan he connected to which switch and sends the building owner a bill for all the wire he used.
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Consider the source. It’s a logic puzzle.
There are fans with DC motors having permanent magents, (e.g. in old cars), but they are few and far between. This has virtually no chance of succeeding.
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It’s a hybrid.
It’s like the light bulb one (check to see which one is hot): it depends on knowledge of information not provided (that the light bulbs in question heat up when in use).
A pure logic puzzle would be something to the effect of, “If Amy is taller than Bob, and Amy is shorter than Chris, which of the three is tallest?” All of the knowledge needed to work out the puzzle is present.
This one is structured so that you have to know something about electricity, or fans, or air currents (or whatever your solution is based upon) in order to solve the puzzle.
Such as: [quote=“roomwithaview, post:125, topic:95184”]
There are fans with DC motors having permanent magents, (e.g. in old cars), but they are few and far between. This has virtually no chance of succeeding.
[/quote]
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It isn’t a tradesman’s exam question, it’s 100% a logic/lateral thinking puzzle. The electrician thing is just an element added to make the story more relatable and less abstract. There’s nothing smug or self-serving about it, and it’s okay to not get most of these (I know I don’t). Think of it like this:
The ‘story’ is just used to wrap up little guideposts and rules into relatable, real-world limitations. The fans can’t be observed during operation, we can be certain which state we put a switch in, we can only check our results once. Everything else (outside of a bad-faith puzzle or esoteric tradesman’s exam) is open, and that’s the happy funtime brain playground part. Don’t get hung up on the narrative of the electrician or wiring code or whatever. Since there are only two states of existence the fans can be in, and two states of existence the switches can be in, but more fans than you could do process-of-elimination on in a single trip up the stairs, there has to be something that a fan does to leave evidence of its operation independent from the switch/observable operation states. The presence/absence of that evidence is the third metric needed solve the puzzle in one trip. The answer has to be something about a fan’s operation that an average person would reasonably know about, or be able to do without researching something specific or owning a multimeter. That’s what separates a puzzle from a test question.
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-lazy electrician looks at switches in basement, pushes buttons at random
-climbs stairs, realizes at each floor that job is too complicated for lazy electrician with poor tools to handle
-climbs to roof
-realizes futility of life
-jumps, dies
-fans spin away or not, because life is futile and we all die anyway
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LOL: We had the same first three lines but then my electrician lights a joint and burns the building down.
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#@thedude541’s answer does work.
Electrician turns switch A on, switch B off, and connects the wire that would normally feed juice outbound from switch C to neutral or chassis ground. He climbs the stairs one time only with his Wiggy or Fluke in his toolbelt, and labels the running fan A and checks the resistance to neutral on one of the fans that’s not running - if it is zero, he labels that fan C, otherwise he labels it B. The last fan gets whatever label remains.
This is a trivial job requiring no parts or supplies, just a normal working electrician’s toolbelt, and does not rely on the fans being either AC or DC.
No, room fans nearly always use AC induction motors, otherwise they’d have sparkly bits like your cordless drill does when you look through the heat vents at the back end. They don’t care about polarity.
DC fans that are reversible do exist, but they are about as common as alcohol-powered fans (which are were a thing) so you can’t rely on them to solve this puzzle.
Simply put, an electrician does not require any assistance of any kind in order to open up a switch box or measure resistance. Just so, a cook does not require any assistance in order to fry an egg.
That’s what he wants you to think.
That’s my Avatar avatar. There are other Avatar avatars on bOINGbOING but this one is mine ;).
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Your electrician’s life is much happier (and longer) than mine!
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Three points:
- If having an electrician’s expertise and/or tools is a detriment to solving the riddle “properly,” not an assistance, then the guy in the riddle shouldn’t be an electrician. Make him the landlord. Or the owner. Or anyone else who might not want to pry the switch plates off the wall as part of figuring it out.
- You can’t assume what an average person would “reasonably know.” I’m a fairly average person, and thought that it was a pretty good assumption that a fan motor, run without electricity, would act as a generator. That, in my mind, is what motors do. I was dead wrong, and now I know something new. We’ve had other answers that rely on “reasonable knowledge” of what fans do: the “check the one with no dust” answer, the “check the light bulb for heat” one, the “reverse polarity” one… most of those have been debunked. Relevant xkcd:
- Finally, to echo @anon50609448, if your instructions don’t make it clear when you’ve found the wrong answer, the instructions are bad.
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but AC fan motors ARE generators … they make AC rather than DC … as I mentioned above if you short out a motor it gets harder to turn …
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If the character’s literal technical skill as an electrician was in play as part of the solution, it would then have to be written by an electrician with specific trade knowledge, and thus solvable primarily only by electricians. It would be a test question and not a riddle. It’s like, hmm… Maybe electricians have a special tool or technique for sensing turning fans from within basements. If the rest of us didn’t know about that device, the riddle is inherently unfair because it requires no solving other than already being an electrician before the question was posed. It also cannot be disputed as an obvious BS answer by the vast majority of folks trying to figure out the answer, making it doubly unsolvable. Being open to new knowledge and seeking out little teachable moments is A Great Thing, obviously, but there are basic constraints to what a riddle/puzzle is. It’s based on clever logic and is not a shibboleth.
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Naw, I don’t think you were. Every time some pedigreed EE has told me that some specific motor isn’t a generator because reasons, I haven’t argued, instead I’ve spun that motor and measured output on the wires.
It’s because cows aren’t spherical, and theory isn’t practice.
The only exceptions to the “every electric motor is a generator” rule are edge cases where there are no magnetic fields involved, such as for example the ball bearing motor, and freshly made AC induction motors that have never been spun so they have no residual magnetism in them.
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Pull out my trusty
Quad copter with camera. No stairs involved at all. How’s that for lazy?
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The point of the puzzle is to realize there is absolutely no way to set three bits of information, get three bits of information in return, and match bit to bit. It simply can’t be done. The intent of the puzzle was to see who is “clever” enough to not get hung up on the fact that they’ve been asked a “logic puzzle” so they can try to think of a practical, real world solution.
So you need another piece of information, some way of telling a fan that has been on for a while before being turned off from a fan that has been off the whole time. In this thread we’ve see people mention that a fan keeps running when it’s turned off, something about dust on blades. Except that ceiling fans I’ve experienced do run for an hour after being turned off. Dust sticks to the blades because they’ve been electrically charged by rubbing against the air (ala balloon against your hair) and doesn’t fall off when the blades are running.
I’m used to seeing this problem with lightbulbs and the “right” answer is that lightbulbs produce heat as well as light, but since the stores are stocking LED lightbulbs, there hot thing is a lot less reliable, and inserting an hour long delay would make it useless.
What is the thing you can tell an hour later about a fan that has been turned on and off that I’m supposed to come up with? Do you know what the answer is meant to be here? I half expect it to be the dust thing and end up being just plain wrong, but even if the answer is perfectly legit, in the end, the solution will be telling me a thing about fans, not a thing about logic.
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Not allowed any help sorry can’t use the landlord.
The fans have only two states, but the fan wiring has more than two states. That’s why you have to be an electrician - you can trivially set the wiring states to on, off, and shorted out which are three separate measurable states.
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And now I know google limits queries to 32 words.
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