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#40 Light bulb moment

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?

Answer next week

Quick Quiz #33

1 Originally named after a brand of washing powder, the axion could clean up what massive problem in fundamental physics?

2 NASA’s Messenger probe orbited which solar system planet between 2011 and 2015?

3 Named after their toxin-firing cnidocyte cells, where might you expect to see animals of the phylum Cnidaria?

4 Toxic acetaldehyde, the most abundant carcinogen in tobacco smoke, can crop up fleetingly in our bodies even if we don’t smoke. Where and as a result of what?

5 Devils Tower in Wyoming – the first US National Monument – is a fine example of what geological feature, defined as an isolated hill with almost vertical sides?

Answers below