# Re: A new Tesla coil and k measurements

```Original poster: "Barton B. Anderson by way of Terry Fritz <teslalist-at-qwest-dot-net>" <classictesla-at-netzero-dot-com>

Hi Antonio,

Tesla list wrote:

>Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
><teslalist-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>
>
>Hi:
>
>I have set up a page showing my Tesla coil with a flat primary,
>where I tried to predict the coupling coefficient in advance:
>
>http://www.coe.ufrj.br/~acmq/tesla/tefp.html
>
>The prediction of the coupling coefficient didn't work very
>precisely (designed: 0.105; measured 0.117), and I am thinking
>that a reason may be the nonuniform current distribution in the
>secondary coil. Errors in inductance calculations would move the
>error in the opposite direction. Do programs as Fantc,
>Acmi, and Mandk take the current profile into account?

Lm is computed for uniform current distribution as would be seen with DC
inductance's. 10% out is too far. The programs will get much closer than
this if dimensions are exact. Myself (as well as Terry and others) have
proven this to our own satisfaction. When you run the programs, use the
programs inductance's to compare the measured to predicted. Because Lm is
computed from Ldc of L1 and L2, you also need to make low frequency
measurements.

My measurement method was to apply 120V 60Hz AC to the primary and ballast
with a 10A load (hair dryer). I hook a DMM to measure current in-line with
the primary. The secondary measurement is also taken with a second DMM to
measure voltage at the top of the secondary. By keeping both meters side by
side, I easily grab current and voltage numbers concentrically. This is
very important for accuracy as load currents can vary from second to
second. I have found that a hair dryer is an excellent load for this
particular measurement. The air passing over the heating elements
stabilizes the load very well. I once made tests using several different
load methods including a 10 ohm 3KW resistor (one of the big boys). The
hair dryer load came out on top for a decent current which remained
constant over time.

If you are measuring at a high frequency, you may want to compare against a
low frequency measurement to confirm a change of Lm at low and high frequency.

Take care,
Bart

```