Re: Magnetic Field in a Flat Spiral

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jim,

On 10 Mar 2002, at 17:23, Tesla list wrote:

> Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jimlux-at-earthlink-dot-net>
> 
> The variation in resistance just due to temperature might have a significant
> effect on Q, even though the temperature variation isn't much. Let's see...
> tempco of copper is something like 0.004 per degree (C).., I think.  A 10
> degree change will result in a 4% change in DC resistance...  It will also
> affect the skin depth, so the effect might be more than that....
> 
> It might also be 0.0004, in which case, we're looking at fractions of a
> percent..

Yes, but doesn't copper have a positive tempco? In which case, had 
that been the influencing factor, I should have scored a higher, not 
lower Q on the second day? And if that's so, then humidity appears to 
be having a greater effect than I had previously suspected?

Regards,
Malcolm
 
> ----- Original Message -----
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Sunday, March 10, 2002 2:28 PM
> Subject: Re: Magnetic Field in a Flat Spiral
> 
> 
> > Original poster: "Malcolm Watts by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
> >
> > Hi Paul,
> >
> > On 7 Mar 2002, at 17:06, Tesla list wrote:
> >
> > > Original poster: "Paul Nicholson by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> > >
> > > David Thomson wrote:
> > > > what is the formula for calculating the Q of a coil by taking the
> > > > two voltages on either side of the resonant frequency?
> > >
> > > Detect the coil's resonance with a distant pickup, or by measuring
> > > the base current.  Turn down the output of your signal generator so
> > > that it is below half of its maximum drive voltage. Tune the signal
> > > generator for a maximum reading on your detector and mark this level.
> > <snip>
> >
> > > A signal generator with a good fine tune is a prerequisite, as is a
> > > frequency counter.  You need to make sure that the drive voltage can
> > > be turned up by exactly a factor of two at the coil base. Use a sine
> > > wave drive voltage.  Even with care the method might only be 5% or 10%
> > > accurate due to the sensitivity to the small difference |f1-f2|.  Day
> > > to day variation of the Q is to be expected, for reasons which remain
> > > unexplored, I think.
> >
> > During a series of precision Q measurements I detected a variation
> > between one day and the next of 6 - 7% in a moderately large
> > resonator. The most notable difference between the two days was
> > humidity. The air temperature on the day that the coil came in lower
> > was also down somewhat - maybe 3 - 5C.
> >
> > Regards,
> > Malcolm
> >
> >
> >
> 
> 
> 
>