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The Correct Tesla Bifilar Coil
James L Kistner Jr

The bifilar, or two wire, wound coil has found many uses and has become very popular lately in applications ranging from the separation of end products from biodiesel to the all popular 'Tesla Coil' (which should not really be described as such but that is not the purpose of this paper). Most of the time the experimenter winding these bifilar coils is under the impression that he/she is winding what is known as a 'Tesla Bifilar Coil' while that is not really the case. Much of the information and instructions for the winding of these coils is not what is described in Tesla's Patent #512340, "Coil for Electro-Magnets". The illustrations below will illustrate the difference between the winding that is being distributed on the internet and the winding that is specified in the Patent (you'll just have to excuse my drawings, I am not an artist).
First, let's look at the two coils as they are assembled and then drawn out into their two wire lengths:

As you can see, the one one the left (distributed on the 'net) is very different from the one on the right, which is the one specified in Tesla's Patent. Does it really make a difference, though? Does it matter which way the bifilar is wound? Tesla thought so and made some pretty nice claims for the coil he patented. If we examine the flow of current in the wire of the coil, it will give a better picture as to why he made these claims. Below is the current flow for the popular 'internet' bifilar coil:

We see opposing current flow in adjacent wires. Compare this to the current flow in Tesla's coil:

Here we have no opposing flow. All current flow is augmented by the adjacent wires, resulting in an increased magnetic field, just as he said. This has recently been demonstrated by an expeimenter who graciously shared his test of the Tesla Coil against a serially wound coil. If it is still available for viewing it can be seen at the link to the video below. Once you see this video, you will probably never wind a bifilar the 'internet' way again.

aka Hermit53


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Comment by julian webb on January 29, 2013 at 1:58pm

Hi, has anyone wound (or thought of winding) a 1:1 air core transformer (forgetting efficiencies etc for now) with the first winding/layer a "Tesla" bifilar primary with a "net" winding/layer on top as the secondary? Just theorising for a minute, would this see a primary pumping out a great magnetic field into a secondary that was virtually invisible to the primary? The secondary would extract energy from the primary without altering it's inductance or anything else? Would the "net" bifilar actually be better at extracting energy that a "normal" winding? Any educated or even off-the-top-of-the-head guesses would be appreciated.

Comment by Gary on July 26, 2010 at 8:21am
Looks to me like if you had one flat coil over another, wired in reverse, it would do the same as the tesla coil, since it's current direction that is creating the emf between wire pairs.
Therefore, the next experiment would be to use x amount of wire to create that with 2 flat coils, vs the same amount of wire in a tesla. I would imagine that you'd be creating a tesla coil this way by dividing the flat coil into 2 and reversing current in one.
Comment by Hermit on July 26, 2010 at 7:38am
Well, Robin, I have wound a coils in the 'Tesla ' patent fashion and did experience just what he said I would..VERY poor induced electricity but very GOOD induction to other coils, etc. As you correctly stated (in you own way) each coil has its own applications, and I believe the 'Tesla Patent' wound coil would be a very good application to pulse systems due to its very small back emf factor (unlike most other coils). The experimenters of the Joule Thief circuit came very close to trying this coil (and had fantastic results) but haven;t as far as I know (my info is of about 7 months ago).
And I would love to test coils for you but, I have only a VOM meter and my calculatons. I don't own one of those (coveted) meters that test the capacitance and inductance :( I have plenty of time, not much energy (and that is very sporadic), but a mind that won't leave me alone :)
Comment by Robin S Hooper on July 25, 2010 at 3:33am
My understand in this is then confirmed, Thanks Hermit. What interestes me is that if you use the right hand rule to indicate the b field direction by way of you finger direction. In the internet wound coil b-field flows into both wires from the outside to the inside and in Teslas' coil one wire flows from the outside to the inside and the other wire parallel to it flows from the inside to the outside. Now this is asumming that the b-field has a circular direction as it flows. In short the internets version flow meshes and Teslas flow into each other. Now thats not to say one is better in design than the other, Teslas' bifiar wind will produce a much stronger b-field. Interesting though. Oh, the video provided, thanks by the way, compares a standardly wound coil with Teslas'. The comparison above how ever does not, but the result are still the same or would it be? Hum, Does any one have the time and the resourses to test the three types? I'm afraid I don't. I have all the wire I need but no instrumentation. Hope I've stimulated some of you. I'll wind, ship to you, you test.
Comment by Mike Hingle on May 24, 2010 at 11:05am
Excellent post ! Short, thorough & to the point.
Thank you.
Comment by skysabre on January 31, 2010 at 7:39am
Thanks for this, Hermit. Thank you very much. This clears out a lot of confusion and disappointments.
Comment by Slaven on January 30, 2010 at 5:18pm
Thanks for this info
Comment by gabet123 on January 29, 2010 at 6:23am
I embeded it for you. Thank you or bringing this to our attention.

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