Re: How to Tune a Flat Spiral Coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Paul, Dave - 

Tesla list wrote: 
>
> Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <paul-at-abelian.demon.co.uk> 
>
> You may wish to measure the mutual inductance as a cross-check. This 
> involves passing a 60Hz AC current through the primary, say around 
> 10 amps, and measuring the voltage induced across one of the (open- 
> circuit) secondary strands.  It should be around 36mV per amp of 
> primary current.

Dave, I would be interested in this as well. I did this with my flat coil. I
recommend a hair dryer as a load (on high ~ 10A) (I've tried beefy 3kW
resistors and all kinds of other loads - the hair dryer has the best consistent
readings - the air stabilizes the heating elements pretty darn well). If you
can hook up a meter for the primary current and another for the secondary
voltage, you can perform concentric readings. This is pretty important for
accuracy. 

For those interested, my flat secondary has a 4.2 turn flat primary 5.2" below
the secondary. 

I measured: M=58.44 uH, K=0.1749 

Acmi v.7b found: 
SCNY-PRMY.M|SCNY-PRMY.K 
   59.00 uH|     0.1765 

How's that for accuracy! 
>
> > I'm using a 15KV 60mA NST. 
>
> Gosh, thats 15kV times sqrt( 6314/9.76) = circa 380kV.  That's an 
> average of 380/227 = nearly 1.7kV per turn!  I'm amazed that your 
> secondary will stand that.  Considering that the actual voltage 
> gradient is almost certainly quite non-uniform, it could be over 
> 2kV/turn in places.  Certainly raises the eyebrow in a Spock-like 
> manner. 
>
> Can anyone claim a similar order of secondary volts/turn from their 
> coils?

Not on any of my helical coils. Haven't gone that far with the flat coil. 

Take care, 
Bart