Puzzle #40: Light bulb moment
5 January 2020
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From New Scientist #3263, 4th January 2020 [link] [link]
A tall office building is being rewired. There is a staircase, but the lift is out of action.
There are four identical-looking wires, A, B, C and D, feeding into a pipe in the ceiling of the basement. You are reasonably confident that it is those same four wires that emerge from a pipe on the top floor. Unfortunately the wires have become tangled, so it isn’t known which wire becomes 1, 2, 3 or 4.
To find out, you can join two wires together in the basement (for example A and C) and you can attach two wires at the other end to a light bulb and battery (for example 1 and 3). If the bulb lights, you have made a circuit.
Starting in the basement, what is the smallest number of light bulb flashes that you need in order to figure out which wire is which? And how many times do you need to climb the stairs?