CR4 - The Engineer's Place for News and Discussion ®
Login | Register for Engineering Community (CR4)

Challenge Questions

Stop in and exercise your brain. Talk about this month's Challenge from Specs & Techs or similar puzzles.

So do you have a Challenge Question that could stump the community? Then submit the question with the "correct" answer and we'll post it. If it's really good, we may even roll it up to Specs & Techs. You'll be famous!

Answers to Challenge Questions appear by the last Tuesday of the month.

Previous in Blog: Refraction Challenge   Next in Blog: Stain: Newsletter Challenge (06/20/06)

Comments Format:


Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.


Rating Vote:


The Mysterious Bell Knot Challenge

Posted June 19, 2006 11:45 AM

The mysterious Bell Knot has been around as long as coiled cords have been around, but was something unimagined by Alexander Gram Bell. What is a Bell Knot? Quite simply, a Bell Knot is when your coiled handset cord develops an irregularity in the symmetric coiling of the cord. It is sort of an anti-coil where the coil direction of the cord reverses. Bell Knots can appear anywhere along a coiled cord as mysteriously as crop circles in a virgin wheat field. While many paranormal events have been captured on film, nobody has been able to capture a Bell Knot's creation. One minute everything is fine, and then you hang up and are horror struck to find your cord has developed a Bell Knot.

Removing a Bell Knot is the Rubik's Cube of cabling. One method is to roll the anomaly, coil after coil, to the end of the cord. However, for this challenge you are not allowed to use this method. After all, Bell Knots are not created this way. How do you do it?

Post Script: Anyone who can provide actual video footage of a Bell Knot's creation can get extra credit for this challenge question.


Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Friend of CR4

Join Date: Dec 2004
Posts: 2016
Good Answers: 35

Here's how I do it

06/19/2006 12:10 PM

I unplug the receiver and cord from the body of the phone. Then I stand on my desk, holding the cord near the RJ end, and let the receiver dangle. The receiver spins for a while and after gravity has run its course, the knots are gone. I then get down off the desk and plug the phone back in.

Off to take on other challenges. Good luck everybody! See you around the Interwebs.
Active Contributor

Join Date: May 2006
Posts: 11


06/20/2006 8:51 AM

Can somebody help me find my "cord" on my cell phone? ;)


Join Date: Jun 2005
Posts: 398
Good Answers: 1

Bell Knot

06/20/2006 2:13 PM

I always assumed that the cause of the knot is the tendency for a person to rotate the head set once each time it is picked up and put down. It would be interesting to see what a phone cord looks like for a left handed person as I know with right handed people, the coils tend to coil tighter and twist together.

If the cord just unwinds when disconnected and left to dangle, that's one thing. But every now and then it gets a reverse curve that looks like one of the coils is deleted (usually you get two of these with a section of regular coils between them). These cannot be gotten rid of by hanging the cord, even with the head set attached.


Join Date: May 2006
Posts: 7
In reply to #3

Re:Bell Knot

06/20/2006 2:59 PM

I looked down to find no fewer than three of these little mistries. They all seen to have formed where ever the cord is doubled over itself like at the lowest point of the hang or where it hangs over the desk edge. It seems more kink problem than a twist problem. Maybe it is a combination of the two.


Join Date: Apr 2006
Posts: 9

Belkin untaglers to the rescue

06/20/2006 5:20 PM

I saw this on the web at site... they should come standard in phone sets! Ever been on the phone with that nagging in-law and had your attention diverted to that old and twisted coiled handset cord? Don't you just wish you could disconnect them and fix that cord? The Belkin Twist Free Adapter will allow you to focus all your attention on the moaning and groaning. With this novel invention, at least your handset cord will never be twisted again. The Belkin Twist Free Adapter attaches to your coiled handset cord and prevents knots, twists or tangles. Our Price $8.40 Availability as of 7:33am 6/9/06 Out of Stock

Hobbies - HAM Radio - New Member Popular Science - Weaponology - New Member United States - Member - New Member Engineering Fields - Mechanical Engineering - New Member

Join Date: Jan 2006
Location: Saint Louis, Missouri USA
Posts: 1932
Good Answers: 9

Bell Knots also occur in Slinky's

06/26/2006 3:09 PM

I have observed that the so-called "Bell Knot" (I never knew there was a name for them!) also occurs in the Slinky toy, a spring steel coil. It occurs when the toy is stretched and gets tangled on itself. In trying to untangle the toy, the loops are sometimes forced to invert themselves, spiralling in the opposite direction. I believe that you will always create an even number of "knots" if the ends are fixed. I just successfully tried this on my phone cord. I was unable to create an odd number of "knots" while the cord was attached to the handset and phone. If you are careful and do not exceed the elastic limits of the material you may be able to pull on the loops between the knots until they are almost straight and allow them to revert to their original orientation.

I believe the "Bell Knot" creation happens because the greatest spring force is in the plane of the coiled loop and only a slight amount in the "twist" which gives the helix its direction. When phone cords and Slinky toys get tangled through stretching, twisting, flopping, etc., the tendency is to pull on the ends to pull the loops apart and untangle them. When they are caught on other loops and cannot untangle in the direction they should, they will "untangle" in the opposite direction since that also allows the coil to resume its shape, acheiving a new "meta-stable" state and creating the Bell Knots.

"What, me worry?" Alfred E. Neuman

Join Date: Feb 2007
Posts: 1

Re: The Mysterious Bell Knot Challenge

02/09/2007 10:57 AM

So, what's the answer? I have one in my phone cord now that I'm anxiously waiting to remove.

Anonymous Poster

Re: The Mysterious Bell Knot Challenge

09/05/2007 6:29 PM

The single-turn instance of this knot can be produced by grasping the coil with your hands a few coil-turns apart and rotating your hands in opposite directions in the direction that "unwinds" the coil. Once you have about one complete revolution, one of the loops near the middle with become enlarged, and then collapse with a single turn of coil folded "backwards" around it's neighbour.

Removal of this type of "knot" is accomplished by reversing the process. Grasp the coil on either side of the knot and twist your hands in opposite directions, such that your hands are acting to "tighten" the coil. The knot will enlarge, the loop with flip over and the coil will collapse back into its un-knotted configuration.

It takes a little experimentation to determine exactly how many turns apart your hands should be to produce/remove the knot.

Over time, a knot of this time can grow if the same anti-coil rotation is re-applied. Each complete rotation will add one reversed coil to the knot, creating a cord that has a number of reversed coils in the middle with a "knot" at each end. Removing a large knot of this time is accomplished by the same technique. Start at one end of the knot, each iteration of the "unknot" maneuver will reduce the flipped region by one loop. eventually, the start and end of the "knot" region will meet, and one more iteration of the maneuver will remove the knot.

Reply Good Answer (Score 2)
Anonymous Poster
In reply to #8

Re: The Mysterious Bell Knot Challenge

02/17/2008 11:32 PM

I am unable to reproduce this. I think a link to a youtube proof is in order...

Anonymous Poster
In reply to #9

Re: The Mysterious Bell Knot Challenge

03/07/2008 1:08 PM

the "good answer" above is total crap. following the method described doesn't yield a true bell knot at all because the direction of the sprial is not reversed. simply pulling the cord will undo the type of kink described.

this is the type of thing that drives people insane. i can find no explanation as to how a phone cord spontaneously reverses its spiral from counterclockwise toward the handset to clockwise toward the handset or vice versa, nor can i find an explanation as to how to undo such a phenomenon. i suggest someone carry one of these bewitched cords into a quantum singularity and see what happens.

Anonymous Poster
In reply to #10

Re: The Mysterious Bell Knot Challenge

06/27/2008 2:26 PM


I studied this for a while and I really didn't believe that some sort of knot-reversal could climb its way up the length of the cord and it turns out that the way it happens and the way to undo it are far simpler.

The best way to learn how to undo the knot is to learn how to create one. Simply find a section of telephone cord that is in its "healthy" spiral state and take hold of one loop with your left hand and the one right next to it with your right hand. Now just twist the whole wire with one hand twisting the loop one way and the other way with the other hand. If you go the wrong way you obviously just make the coils smaller and tighter. You have to go the way where it starts getting wacky and it will eventually pop into that dreaded knot that has driven us all crazy! Just grab the coils on both sides of the knot and twist in the opposite direction to go back to normal. yay! :)

Anonymous Poster
In reply to #11

Re: The Mysterious Bell Knot Challenge

08/25/2008 11:32 AM

I found you can just take the phone cord and wrap it FLAT around your finger (so it looks the same as a 'healthy' section of cord) starting right before the section where the knot is located, and keep going past the knot until things look normal again - then remove your finger. I just removed two of these annoyances in my spiral phone cord.

Anonymous Poster

Re: The Mysterious Bell Knot Challenge

09/22/2008 2:16 PM

Being of a devout Repub persuasion I think it must be a Democratic conspiracy. Maybe this election is gettinig to me. I just kept twisting the cord and worked the knot down to the end.

Anonymous Poster

Re: The Mysterious Bell Knot Challenge

06/09/2009 6:27 AM

These knots appear when you twist the cord so much that it finds another stable position by binding itself. The most clever way to solve this problem is to unplug the cord then twist the cord where the knot is. To be sure that you twist the cord in the right direction, you'll notice that the cord will make a precise reversed "U" shape. You just have to continue the twisting until the cord gets back to its original position.

Reply to Blog Entry 14 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".
Copy to Clipboard

Users who posted comments:

Anonymous Poster (7); archeryguru2000 (1); Chris Leonard (1); davemos (1); Howetwo (1); kbinnj (1); STL Engineer (1); wchaffee (1)

Previous in Blog: Refraction Challenge   Next in Blog: Stain: Newsletter Challenge (06/20/06)
You might be interested in: Coiled Cords and Cables, AC Power Cords, DC Power Cords


New Privacy Policy

We have adopted new policies. Please read each one carefully.