Re: Dimensions of my flat spiral coil

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

> > (more or less) from each other.  This resulted in three different
> > inductances in each of the three wires.  The inductances were 6.20mH,
> > 6.23mH, and 6.25mH.  Oddly enough, each of the three wires had pretty
close
> > to the calculated inductance!
>
> Now this sounds correct. Each wire is one width apart and have different
radius
> (slight, but differnt). This tiny variable will stack up for the number of
> turns. I would expect to see 3 seperate inductances slightly apart.
>

Can you measure changes of <0.5% reliably?  I am reminded of the problem
with inexpensive DMMs... It may have 3.5 digits on the display, but only be
accurate to 5%.

Off hand, the relative values between windings are probably more accurate
than the absolute value, especially since they are all about the same value.

What frequency are the measurements being made at?  What sort of
surroundings and what was the test lead configuration (the test leads
themselves could easily have 10 uH of inductance, or more important, the
uncertainty in the test lead inductance is on that order.  A measurement
uncertainty of 10-20 uH would be on the order of the differences you are
seeing.  Parasitic capacitances in the pF range might also have a
significant effect, depending on measurement frequency.

Not to be nit picky here, but I've gotten "beat up" in reviews for seemingly
simple measurements where all these effects weren't at least quantified, if
not measured.  Precision metrology is much an art, and remarkably
painstaking, even for simple measurements.


For all intents and purposes, if you told me that the three windings had the
same inductance, your measurements would support that.  Interestingly, if
you hook the windings in parallel, since they are very tightly coupled, I'd
ballpark the resulting inductance at 9 times that of the single winding, or
around 50-60 mH, just on the N^2 principle.