## Wednesday, July 11, 2007

### Discover the wrong wire Puzzle

Between two telephone exchanges about 120 wire cables are laid underground. The telephone exchanges are 10 kms are apart.
After the wires are completely laid, someone realized that some of the wires are of the wrong type. None of the wire is labeled and cannot be distinguished visually. And hence connections at both the telephone exchanges is not possible.
You are a technician and you are asked to label the wires after identifying without ripping them all. You have a battery , bulb , pen to label, and tape with you and no transportation.
What is the shortest distance you have to travel (walk, hope you ll want it to be the shortest) to identify all the wires and label them?

1. short all the wires at 1 end using the tape and walk over to the other end
there, using the battery and bulb, determine the resistance of all wires. wel have 2 unknowns R1 and R2 - resistances of the two types of wires
pick any 2 wires and note how much potential difference is required to have some fixed current....

then wires on this end can be labeled. now short these and go to the other end and do the same thing

2. There is no way to measure the resistance. See the original problem specs. My solution comprises of 3 trips which is 15km.

3. Er ah whoops! Maybe I should read more carefully too. 3 trips would be 30 kms in this problem.