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.