The Wiring Problem

This is a problem that I borrowed from the engineering magazine, "The Bent". Of the first 15 people I showed it to, only one person besides myself solved it correctly. It's not that it's that tough, it's that most people don't care. Here it is:

You are an electrician on a construction project. A cable has been buried between two buildings a mile apart. The cable contains 15 individual wires, which are indistinguishable -- all the same color, thickness, type of insulation, etc. They are also tightly bound together by the outer cover and cannot be stretched or slid by pulling.

Your job is to electrically identify each exposed conductor at both ends so that, for example, the end with the tag "#1" is connected to the other end with this tag a mile away. Likewise for wire #2, etc.

You must complete the task by yourself. Your only tools are a working light bulb, fresh battery, and enough tags and writing materials to identify all the wires. You also have sufficient electrical leads to connect the bulb, battery, and any number of wires, in any combination. However, the combined length of all the leads is much less than the distance between the buildings. Also, all wires and leads have negligible resistance.

What is the fewest number of trips required between the buildings in order to fully identify each individual wire at both ends?

Comments and suggestions welcome, at marzolian@yahoo.com
Complaints cheerfully ignored. Last revised November 13, 1998.
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