My forman gave me this question . Maybe he just play'm with me but here it goes:

You have 5 wires run from the basement of a building to the roof. Your on the roof. You can go to the basement and come back to the roof only once. How can you use a continuity tester to determine which wire in the basement is the same wire on the roof. In otherwords, how could you ring out all five wires by only going from the roof to the basement and back to the roof.
. -Erik

use 5 resistors?

Erik,

It's quite possible to identify the wires with just a continuity tester and one trip.

You'll probably need to go get a pencil & paper to follow along...

Starting on the roof, link together any two of the five wires. Now select another two of the remaining three wires and link those together. Identify the remaining open wire as #1, then start your journey down to the basement.

Now, at the other end of the cable, start probing with your continuity tester until you find a pair of wires which complete a circuit. Label these wires #2 and #3 (it doesn't matter which way round; we'll sort that out later). Now find the other pair of wires which gives continuity and label those wires #4 and #5. By default, the remaining wire must be #1.

Thus far, we have #1 identified at both ends and the other four wires identified to the level of two pairs.

Still in the basement, link #1 to #2. Link #3 to #4. We now have all five wires daisy-chained up and down the building. Go get a cup of coffee to fortify yourself for the climb back to the top!

Back on the roof, remove both links you made earlier, but remember which wires were paired by the links. Now, out with the tester again and find which wire has a circuit to #1. Identify this wire as #2. The wire which was previously linked to #2 must, logically, be #3.

Only two to go now. With one probe on #3, find which of the two remaining wires has continuity and label it as #4. The last wire must be #5, of course.

This method has identified the wires and proved that all five have continuity from roof to basement.

Ouch my brain!

Ok, your on the roof.

Go to the basement and get the walkie-talkie from your buddy in the basement leaving him only one.

Go to the roof and sort it out over the walkie-talkies.

Thats only one trip too. HA!

First read them for voltage or (lick the wire, only for expert electricians or bosses). No voltage, check continuity, take your first trip downstairs and hook all wires together and check continuity then you will know. After that check for branch circuits.

[This message has been edited by Resistor (edited 06-13-2002).]

Hey, I thought of something like that too! Connect one wire to 120V, another to 277V, etc, then go touch the other end and see how high you jump!

If you don't mind though, I'll stick with my other method.

Paul, all he said was check for continuity! As far as I'm concerned I think I did that. Would'nt be the first time I was wrong.

Sorry, it was the "lick the wire" part that brought out my sarcasm!

Yes, I would certainly check for any existing shorts or connections between wires before starting as well. I agree that your suggestion would check the continuity of all five wires, but I took the problem to be much more than that.


I took this to mean that the problem also entailed identifying the wires at each end.

Isn't this what you meant Erik?

Pauluk,


WOW, that was smart. I had to draw out a picture for that one. But sure enough, that works.

Here was my thinking:

on the roof: tie 3 of the 5 wires together. ground out one wire. leave the last wire open.

down in basement: identify ground wire as having continuity with ground. identify open wire as having no continuity. The other three wires will read continuity between them. tie the identified ground to one of the three remaining unidentified wires. and ground out another of the unidentified ground wires.

Back up on the roof: Identify which of the three wires is open, grounded, and tied to the original grounded wire.


Of course, a walk-talkie might work better.

Yep, that would sure work for five wires.

The advantage of the pairing method is that it can be extended to any odd number of wires and you don't need a ground connection, although after about 9 or so it starts to get the gray-matter in a bit of a spin trying to keep up with all the links!

If you start with an even number of wires and access to a ground connection as you described, then you can follow through in a similar way.

Here's a follow-on puzzle for you. How could you test and identify six conductors, again using only a continuity tester and making one round trip to the basement?

Only this time, you're not allowed to use a ground connection at either end as part of the procedure. Neither are you allowed to use batteries, AC voltages or anything else like that. Just your continuity tester and linking the wires at each end.

Oh, and no assistants with walkie-talkies, either!

P.S. Hint: Start by combining an element of your description with an element of mine.



[This message has been edited by pauluk (edited 06-15-2002).]

Pauluk,


I just got your last challenge. My brain is rack'n. I'll work on it. Have to leave soon. I would think you'd link on the roof:1to2, 3to4, 5to6. Then in the basement identify those loops. And then link together 2to3 and 4to5. now its one daisy chain. let me work from here, I'll write back later.-Erik

Erik,

You're likely to run into problems starting off that way; try to follow it through and you'll see what I mean.

The trick is to be able to positively identify one conductor at both ends to provide a reference point. With only one round trip allowed, you must be able to identify that first conductor from down in the basement. If you start with six wires all linked into three pairs, you won't be able to do that.

I love these sort of brain teasers, as they're a great exercise in logical thinking AND have a practical application.

Keep working on it!

Paul of the UK
That was fantastic, now that is logic in its purest form.
As a side note, I was watching the History Channel, I think the title was "Science and the Swastika", during the war we and the British were worried about the Germans coming up with an A-bomb first. Unbeknownst to us the Germans had it Bass-Ackwards, but two British Scientist came up with the Critical mass of 2 pounds, Which worked. The Germans had this idea of it being huge amount of Uranium which no one could even contemplate...
anyway great logic

-Mark-

Mark,

I've never heard about the A-bomb thing. We have a History Channel on satellite over here; quite fascinating at times.

I claim no originality for the wire ID problem. It's just one of the tricks that you pick up working as a telephone engineer.

The modern phone cable is pretty much foolproof with each pair identified with both colors, e.g. pair #1 tip=white w/blue bands, ring=blue w/white bands.

But the older style cable just used solid colors on each twisted pair. So if someone managed to untwist the pair (sometimes while trying to be "helpful" !!!), on a 25-pair cable you were left with 5 plain white wires, 5 blue, 5 green, 5 red, etc.

Often it was possible to apply tones or other test signals, but occasionally there was nothing for it except to trace out using just a continuity tester. If you were working alone, tricks like this saved a lot of legwork.

Paul

Well ever since I read you first post I have been keeping my continuity tester handy.
A method like that comes in handy for sure.

Mark

can we assume a notable increase in R as a 'loop' is metered in increments?

Ok, here it goes:

On the roof:

mark one wire #1. Pair up two wires and label them 2,3. Pair up another two wires and label them 4,5. Mark the last wire #6.

down in the basement:

identify the two loops and label them 2,3 and 4,5. identify each of the single wires as 1,6. Now mark all your wires individually. So 1# of 1,6 and #2 of 2,3 and #3 of 2,3 and #4 of 4,5 and #5 of 4,5 and #6 of 1,6.

Now, link together 1,3,4 and then link together 2,5,6.

Back up on the roof:

#1 will have continuity w/3,4
#6 will have continuity w/2,5

untie 2,3 2 will light with 5,6 and 3 will light with 1,4

untie 4,5 4 will light with 1,3 and 5 will light with 2,6.

did I get it?

Sorry, but if I've followed your proposal properly, it wouldn't work.

Let's see if I've understood you correctly.....

On the roof:
mark one wire #1. Pair up two wires and label them 2,3. Pair up another two wires and label them 4,5. Mark the last wire #6.
O.K., so you've positively decided upon the identification of #1 and #6 on the roof, and you have the remaining wires looped as a 2/3 pair and a 4/5 pair, and so identified. Right?

down in the basement:
identify the two loops and label them 2,3 and 4,5.

But how would you determine which is the 2/3 loop and which is the 4/5 loop? Or do you just mean at this stage to say "one of these pairs is 2/3 and the other is 4/5" ?

identify each of the single wires as 1,6.
Fine, you know that one of those open wires must be #1 and the other must be #6.


Now mark all your wires individually. So 1# of 1,6 and #2 of 2,3 and #3 of 2,3 and #4 of 4,5 and #5 of 4,5 and #6 of 1,6.

Problem: You've already decided upon the identity of #1 and #6 up on the roof. How would you know which is which in the basement in order to label them?

I can see your reasoning in the remainder of the procedure, but it's based upon a 50% chance of getting the pairs right and another 50% chance of guessing the ID of the open wires correctly.

You're kind of trying to identify the single wires using the pairs and identify the pairs using the single wires at the same time.

Starting with two looped pairs and two open wires on the roof means that you have no way to accurately identify one wire to use as a reference point while in the basement.

Back to the drawing board???7



[This message has been edited by pauluk (edited 06-20-2002).]

Assuming no shorts and wires are continuous.

On the roof:
Arbitrarily label one wire as #1. Tie three wires together and then tie the remaining two together.

In the basement:
Mark the wire that did not ring out as #1. Find the bundled wires and note the groups. Arbitrarily mark one of the wires in the three group as #2 and tie that to #1. Mark the second wire as #3 and leave that hanging. Mark the last wire as #4. Then take the two bundle and arbitrarily mark one of the wires as #5 and tie that with #4. Mark the last wire as #6.

On the roof:
Untie the three group. Ring out using #1 to look for #2. Ring out using the two bundle to look for #4. Remaining wire of three bundle is #3. Untie two bundle and use #4 to look for #5. Remaining wire is #6.

Hi Bazooka,

Yep, that's the method! Did you get it Erik?

As with the example above, you can extend this one by pairing up extra wires into the daisy-chain. The group of three and the single open wire gives you the reference point that you need.

To comlete the picture, the "odd man out" is the case of 4 wires. You can solve that one thus:

On roof, link two wires, leave other two open.

In basement, identify open wires and label them as #1 and #2. Identify looped wires as #3 and #4. Tie #2 to #3.

Back on roof, ring out from looped pair to identify #2. Other open wire is #1. Remove the tie, identify #3 using #2, then last wire is #4. The only "problem" with this one is that unlike all the others you haven't confirmed continuity on all 4 wires (#1 hasn't been tested).

It's sometimes easier if you have a ground reference available, but by just using a continuity tester it's possible to correctly identify any number of wires from 3 upwards by these methods.

Sparky,
The resistance angle could be useful as well. I guess I should've specified an audible continuity tester only, no ohmmeter!

One last challenge:

How could you correctly identify just 2 conductors? You can use a continuity tester or ohmmeter, you're not allowed to use a ground reference or any other cable, but you may use one additional component.

Oh, and you can start on the roof and by the time you've finished testing in the basement, the wires should be correctly labeled at both ends. No return trip!

Start at the roof and install a diode between the two wires. Label each wire. Use the continuity tester and record the readings (1-2 and 2-1).
Make the trip to the basement and make the same checks.

Yep, that's what I had in mind!

Now, how would go about identifying just a single conductor?

Just kidding....

neat thread