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Re: A Little more than general questions II



Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 4/13/01 6:53:20 AM Eastern Daylight Time, tesla-at-pupman-dot-com 
writes: 

Hi Wade! 
(interspersed comments) 


>
> Hi guys, 
>
> Again , Thanks for all the input on my last questions. 
>
> I'm wondering if one of you has a formula for calculating the 
> inductance??? The one I have requires that the permeability of the core 
> be included. And since this device has an air core............maybe the 
> permeability is  1???



Quick & Dirty formula for air core, single layer coil:  L=(rN)^2/(9r+10h) 
L in uHy, r=radius in inches, N=number of turns, h=height of winding in inches 
For a flat primary of inner radius ri and outer radius ro: L=(RN)^2/(8R+11B) 
where R= (ri+ro)/2 and B=ro-ri. Again, N= number of turns, L is in uHy. 

>
>  I am also working on another project that I requires me  to calculate 
> the inductance of my coils, but these coils have iron and steel cores, 
> and permeability information must be top secret or something, cause I've 
> been unable to locate it. 
> Anyway, back to TC's 
>   I had previously asked how the diameter of the secondary affects the 
> operation,.......and I think Greg you mentioned that the operating 
> frequency will decrease with an increase in diameter??? Now the Tesla 
> coil is very dependent on the frequency being high??? Correct??? So, 
> increaseing the diameter will increase the "Q" point, which is a bad 
> thing, the lower the Q the closer to (theoretical) true resonance??? 
>   I'm hung up on this diameter thing because I have a 6 ft piece of 12" 
> diameter PVC, and I'm debating to whether or not I'm going to build my 
> first coil with it, and it will be a dandy. Using #22awg -at- 1,000 turns 
> will take up about 2 1/2" feet. I'll probably save that piece of plastic 
> for a later coil. 



In general. larger coils do have a lower resonant frequency than small coils, 
since inductance increases with radius. This lower frequency is still in the 
50-100 Khz range- not exactly "low frequency AC". People with 12" coils can 
lead meaningful and productive lives. ;-) 

>
>  It seems that the spark gap needs to be adjustable while the coil is in 
> operation. 
>
> What are some SAFE methods of constructing an adjustable spark gap??? I 
> have some ideas, just looking for other input. 



If I needed a spark gap that was adjustable under fire, I would be thinking 
an electrode holder on a threaded shaft connected to a looong plastic shaft 
extender, connected to a low RPM gearhead motor (~4RPM) with a pushbutton 
switch. In general, if your hand is close enough to the gap to make 
adjustments under full power, then your head is close enough to the secondary 
to take a strike.:-P 


>
> And also, what are some reliable materials that can be used at the spark 
> gaps terminals???  Brass??? Carbon??? Water??? Something that won't melt 
> down. 



Something that won't melt or erode, is like a razor blade or drill bit that 
never gets dull. There ain't no such animal. Tungsten will probably erode the 
slowest, aluminum the fastest, with other stuff in between. If you are really 
on a tight budget and don't want to use brass, try liberating the contacts 
from a number of old distributor caps. (from the junk yard, NOT the parking 
lot) ;-))) 


>
> Thanks again, 
>
> Wade 



Hope this helps, 
Matt D.