I was looking at the Tesla patent about the bifilar coil, pretty creative arrangement of the coil turns. Overall, it lowers the self-resonant frequency of the coil (when compared with a simple/monofilar spiral coil), which is pretty cool! 
https://patentimages.storage.googleapis.com/0b/3c/ab/eb51aebe935f8c/US512340.pdf
No George, it's not cool, it's a very good way to make a poor inductor into a very crappy inductor, what use is it outside the addled minds of Tesla loving, pigeon molesting, deluded morons? Why would you want to reduce the SRF like that? You would spend the rest of your life trying to hit the resonance that you were aiming for.
And what a way to tank the Q of your tank!
Oh, so you are a Tesla hater, too? I've heard that recently, that it is trendy to bitch about Tesla. I've noticed the trend started about the same time when the anti-Musk campaign started, which has a Tesla factory, so if dude Tesla was shit, Tesla brand will be associated with shit, too, and since Musk name is tied with Tesla cars, Musk will be seen as shit, too.
OK, whatever. But you are insinuating that I'm a "deluded moron" because I like the elegance of his patents. Not nice. Bad Xena, bad! Don't try to bully, or to ridicule others, because by doing that you are only diminishing yourself. I've noticed you turned personal attacks into a habit, in the dodgy tech section. You and the other 2-3 friends of yours made yourself a small bullying-gang. You prop each other by thanking to each other to the most unspectacular replies, as long as those replies are aligned with your little bulling gang. You guys are making a disservice to this forum, because you deter others to discuss openly.
So, bad Xena, bad! Same to other 3-4 guys that jump on any shoddy tech topic to coerce others instead of pointing the mistakes. Then if your coercion fail, you start personal attacks, making laugh of new users, talking rude, and then inviting them to never come back again.
Kind reminder again, we are here to talk about tech, not about people. Tech is nice, people, well... some are not nice, so let's keep it about tech.
Now about your tech questions. I guess hating the dude Tesla stopped you from reading the patent, because the patent title + the first paragraph answers your questions:
- what use is it outside the addled minds of Tesla loving, pigeon molesting, deluded morons?
The answer is in the patent title "COIL FOR ELECTROMAGNETS", a patent from 1894
https://patentimages.storage.googleapis.com/0b/3c/ab/eb51aebe935f8c/US512340.pdf- The problem was to push big AC currents through a coil with many, many turns (e.g. for an electromagnetic crane)
- Typical solution back then (cca. 135 years ago) says the patent, was to add a capacitor to the coil, so the capacitor reactance will cancel the coil reactance (at least partially) and thus the power source will see the resonant LC as a small resistive load.
- The resonant LC tank was more convenient than the L alone, because It was easier to put a lot of AC active energy into a small resistive load, than it would be to put the same active energy in an L with with a lot of reactance.
- An additional capacitor was expensive, so the idea was, why not using the parasitic C in the turns of the coil? (we are still talking about a normal spiral coil, not the bifilar coil arrangement proposed in the patent). The parasitic capacitance of the coil was too small to form a resonant LC for whatever frequency they use to power the electromagnet back then.
- And here comes the clever/elegant solution proposed in the patent: Instead of using a coil in the shape of a normal spiral, like in fig. A in the patent, let's make a spiral coil by using only half of the wire. With the other half of the wire, we made a second spiral, interleaved with the first spiral. And we connect them like in fig B.
That way, the average voltage seen by the parasitic capacitance between 2 consecutive turns is half the voltage in fig B, which is much bigger than the voltage seen by the parasitic capacitor in the normal spiral in fig A (on average, total voltage divided by the number of turns).
The energy stored in a capacitor, is proportional with the square of the voltage. So now, because the voltage is much higher, the parasitic capacitance will store much more energy than the (about the same) parasitic capacitance of a normal spiral.
I find that interleaved arrangement very clever. It "enlarges" C, more precisely the energy stored in the form of the electric field, by simply rearranging the turns. It is like we would now have an LC tank, but in which the C is somehow seeing an "amplified/larger" voltage. This last paragraph is my own interpretation, not in the patent.
Don't know how to say it, but to me, that patent is certainly a very clever and very elegant by its simplicity. By contrast with your reaction, I don't have any emotional response when thinking about N. Tesla. But if you hate Tesla that much, whatever, imagine the patent's author was Edison, or Westinghouse.
