Puzzle: the lazy electrician

Originally published at: http://boingboing.net/2017/02/15/puzzle-the-lazy-electrician.html

three candles lit at the same time, one for each room. The fans are turned on and off in 5 minute cycling through each fans and the candle length (or resulting apartment fire) determines the switch association.

  • turn one on
  • go up the stairs
  • see which one is on - you got one
  • while upstairs, pick one of the ones that’s off, disconnect the wires from the motor
  • go down the stairs
  • turn on one of the other two, slap a multimeter on it and see if the electricity is flowing - you know which one it is, and by default which one the other one is

what’s that you say, he has to go back upstairs to reconnect the one he disassembled? - nope - he wasn’t asked to get all three going, only to figure out which was connected to which switch - did that - one of these days, when he has to go back upstairs for something else, he can reconnect it


The electrician disconnects the power, rewires the fan’s lines to be a straight connection, then climbs the stairs to install switches in each apartment, then returns to the bottom floor to hand an invoice to the idiot who owns the building, and warns them about getting the rest of the building up to code, to avoid a fine.


If the solution is the same as the three light switches puzzle, it is pretty simple. The key to that one is to know that light bulbs have a lingering property when they have been on for a few minutes and then turned off. A potential further complication here is that one may not know the initial state of a fan that turns on/off with a button. If we can presume that the fans are all off at the beginning, the solution is not so difficult. If the beginning state is unknown, I am not aware of the solution.


My guess is this

Turn two fans on, let’s say A and B, and let them run for about an hour. Then turn one of them off, let’s say B. Arguably, the room with the third fan, C, the one that never turned on, will be the warmest room. So A will be the room with the running fan, B will be the cool room with no fan running and C will be the warm room with no fan running.


Does a fan actually lower the temperature of a room, or does it just move more air to allow the body to feel cooler due to more effective evaporative cooling, aka sweating?

If they are typical American ceiling fans (as pictured) with light kits, it could be quite simple, and essentially the same as the three light switch problem. Perform the same steps as you did. One will be on. That one is easy. Of the other two, one will have a warm light cover - that is the one cycled on/off. The cold one is the one left alone.


It takes him nearly an hour to climb the stairs. Will the difference in warmth still be detectable?


refuses to use any assistance of any kind

Pick one.

Also it never says they have access to the apartments.


Excellent point, probably not. I’m no electrician, which should be obvious from my next suggestion. Maybe he can short one out and blow the bulb or fry the motor?

The electrician disassembles the fan switches and attaches color coded pairs of wires to each. Then he spools the wire pairs up to each room, and takes turns connecting them to see which starts the fan.


The electrician runs two fans for awhile, then turns one off and heads upstairs. He then can tell which of the fans is on, which is off, and which was never recently turned on.

How he tells which fan was never on is beyond me. Maybe the room is cooler? Maybe the fan is warmer? Maybe the papers and dust and whatever are more scattered than they were? I don’t know. This electrician may be lazy, but he’s far more observant than I’ll ever be.


If it was DC power, the electrician could rewire one switch so it would turn the fan backwards. But that doesn’t work for a typical AC installation.

That said, nothing else about this installation seems typical…


He uses the correct tool: a remote continuity tester:

“allows a single user to identify up to three wires or cables at a time”


Push button A.

Leave button B unpressed.

Dismantle button C and hook the leads directly up to a 220V utility main.

Go upstairs and find: one fan that’s on, one fan that’s off, and one fan that’s on fire. Map those to buttons A, B, and C, respectively.


This is really close to what I came up with!

Go into the apartments and take the owners’ electric clocks and drape their cords over the fan blades. Synchronize the clocks to the electrician’s wristwatch.

Go to the basement. Look at the watch; turn on Fan A and wait five minutes, then turn it off. Wait five minutes doing nothing much (lazy, remember? Roll a smoke!) then repeat the process for Fan B and C.

Trudge upstairs and look at the times the clocks recorded before their cords were yanked out of the wall and they were whirled around the room and smashed through the curio cabinet.

This might be a little bit of improvement over the candle trick because it’s slightly less likely to set the house on fire, and you don’t have to wait as long between switches (because electrician’s candles burn very slow, so you’d have to wait around 15 minutes or so to get a reliable visible difference in length).


Look bro, I get that a gun wielding wizard is inherently illogical, but c’mon now.

That’s just crazy.


It doesn’t say he doesn’t come downstairs again. So I say he goes to the top floor, ties strings to two of the fans, spools the string to the basement, and sees which button causes which string to be wound in.


Yeah, I know. All clocks are battery driven and electroluminescent now… nobody has corded analog clocks other than me… but it would have worked back when everybody had a telechron!

Edit: @bobtato has it! All electricians have a bucket or two of pull cord, string is the indicator to use, not candles or clocks. Electricians and plumbers rarely carry candles any more… they have flashlight apps on their cell phones instead.


The electrician turns all fans on for a few hours and then turns them off. She waits 1 year for a lot of dust to settle on the tops of the fans, turns two on for a while, turns one off, goes upstairs. Not moving fan with less dust is switch that was on then off. moving fan is switch that’s on, not moving fan with more dust is switch that was off.