# Resonating Coils

1. Oct 26, 2004

### Jdo300

Hello, I have kind of an odd question here. I was wondering how to make an electric coil that will naturally resonate at a perticular frequency. I am a novice when it comes to the associated physics so I'm hopeing that someone can explain this to me.

I also have another related question. I heard somewhere that the earth has an electromagnetic field that resonates at a perticular frequency (I've heard the phenomenia referred to as Schunmen Resonance). Is this in deed true? If so, I wanted to try and make a coil that would resonate with this 'natural' frequency. Any help is appreciated.

2. Oct 26, 2004

### pervect

Staff Emeritus
There's tons of crank sites about the Schumann resonance, the only halfway decent non-cranky web reference I found was

here

Since it's at 7.8 hz, you would need a very large coil and a big capacitor to construct an LC resonant circuit.

the resonant frequency will be 1/(2*Pi*sqrt(L*C))

You will want 2*pi*f*L to be >> than the resistance of the coil to have a reaonably sharp filter (high Q).

You'd probably be better off with other methods than a physical LC filter if you were seriously interested in studying the phenomenon though. (You can build active filters with op amps at this frequency, you could probably even sample the waveform and use a computer to due the filtering / analysis numerically). Either of these methods will require non-trival electronics knowledge though.

There are some formulas for the inductance of a solenoid

here

as far as your original question goes. You would probably wind the inductor yourself, but buy the capacitor, if you proceed with your original plan.

3. Oct 27, 2004

### Jdo300

Hi,

Thank you for your reply. I am wondering, is it possible to build a coil that will resonate at the frequency without the use of a capacitor? And how big a coil would one need to be able to resonate at this frequency? I would ideally like to make it as small as possible but I could use different gagues of wire and such if that would help. Also, would putting the coil on something like a ferromagnetic toroidal core help any? I'm wotking on an oddball project and thats why I mentioned the toroid. The coil would only be wound on part of the core, not all of it.

4. Oct 27, 2004

### Integral

Staff Emeritus
No. An inductor needs a capacitor to form a resonant tank.

Sorry, thats just the way it works.

5. Oct 28, 2004

### Jdo300

Hmmm.. that makes sence I suppose. But just to make sure that I understand this, would a capacitor be needed if the source causing the coil to resonate was from the air (like a radio wave)? THe setup I am making would not need to amplify the signald but just get the coil and the toroid to oscilate back and forth with the frequency of the source. Is it possible to have a sort of inductive oscilator where the only thing realy making the coil do anyting is the already oscilaring resonance energy in the air? I'm sure I'm probably explaining this wrong. BUt ultimately, I'm trying to do an experiment where I have a toroid core and a coil that is tuned to resonate at the 7.8Hz frequency, not because of any electrical energy that I supply to the coil but jus from the radio signald in the air causing it. My next test was to see what would happen if I stuck a magnet up to the coil or just somewhere on the toroid core (which would be made of a ferromagnetic material). I heard somewhere that doing this causes the coil to resonate at a higher frequency like a circulator (which I am still learning about).