Author Options:

Waveform of pulsed magnetic fields... Answered

I have recently been astoundingly interested in magnetic fields, for seemingly no reason. The point is, that in reading up on it, and being a big fan of the greatest mind of all time, Nikola Tesla, I have been stumped with a question that I can't quite seem to answer, and can barely wrap my head around. My question is this; when one takes a normal electromagnet and sends pulses through it, is the resulting pulsing field considered to be longitudinal or transverse waves, or something else all together? This has baffled me, and I am curious to see the answer. Another question I have is, what exactly does Tesla mean by "hertzian" waves, I'm assuming that that his "non-hertzian" waves are longitudinal waves, but I could be wrong.

A little background on the question... This question arose when I was looking at the 'ible "spooky tesla spirit radio"  (something along those lines, I'm not sure the exact name). I read somewhere in the comments section that Tesla found Hertzian waves to be, for lack of a better term, a waste of time. This brought the question of different waves up, and one thing led to another and here we are!!

Thanks for any and all answers!!



Best Answer 8 years ago

"Hertzian waves" are just radio waves (named after Hertz, who discovered them). They are always transverse, as are light waves, microwaves, x-rays, and any other EM radiation. You can't do longitudinal waves with the electromagnetic field, because the induction equations are necessarily transverse. When you pulse an electromagnetic, you get a wave packet -- multiple frequencies which sum up and interfere with one another to produce an approximate square-wave envelope. But they're all transverse.

Ok, that makes sense, and thats what I thought on the "hertzian waves". So I guess the real question would be how does one create a longitudinal wave in such a way as Tesla. If I'm not mistaken, this is a BIG question that is still unanswered... Or am I mistaken there too?

True "longitudinal" waves (that is, where the oscillations are parallel to the direction of motion, as with sound waves in a gas) are simply impossible with the electromagnetic field. You can show that straightforwardly from Maxwell's equations. So either Tesla was spouting nonsense (which is eminently possible), or he meant something different by "longitudinal" than we do in physics today.

Yes, in doing more research for hours throughout the day, I have discovered that, it seems at least, what he was referring to was not the actual propagation of the magnetism but the force driving the magnetism in the actual wire. It seems that he was referring to the movement of electrons in the wire, of course AC, in the single wire that causes the pulsing magnetic field. The electrons build up and move forward along the same way that the oscillations are moving, therefore this is similar to a longitudinal wave. The article I read did a much better description, as what I said wasn't exactly right, or necessarily true, but this how I understand it.

Okay, that's not an unreasonable description of (semi-classical) electron flow in a wire. The electrons effectively "diffuse" under the pressure of a voltage difference. In an AC system, that voltage difference switches direction periodically, and you can easily imaging the electrons forming periodic density structures (like pressure waves) as a consequence. That's not what "really" happens, but it's a perfectly sensible picture. Thank you for the information!

Oh ok well that makes sense! Thank you for all the help and discussion!