# RE: K Factor & Mutual Inductance

```Subject:
RE: K Factor & Mutual Inductance
Date:
Fri, 11 Apr 1997 07:35:27 +0000
From:
"John H. Couture" <couturejh-at-worldnet.att-dot-net>
To:
Tesla List <tesla-at-pupman-dot-com>

At 03:33 PM 4/10/97 +0000, you wrote:
>Subject:
>        RE: K Factor & Mutual Inductance
>  Date:
>        Thu, 10 Apr 1997 07:54:36 -0400
>  From:
>        Heinz Wahl <hwahl-at-jtc-campus.moric-dot-org>
>    To:
>        "'Tesla List'" <tesla-at-pupman-dot-com>
>
>
>John,
>
>        You state that the R is effective, not DC or AC. Why can't you
>measure the R of the coil, calculate the XL, at the
>expected frequency of operation, and use the result to find a ballpark
>Q? Could you elaborate on effective R?
>
>Heinz
>
>>---------------------------------------------------------

Heinz -

When you say "measure the R of the coil" I ibelieve you mean the DC
measurement. Many coilers calculate the Q factor as you describe and
obtain
Q factors in the hundreds and sometimes thousands. These high Q factors
can
not be verified by tests that are properly performed.

The best way to get the calculations and tests to agree is to build
many
different coils and test them for Q and XL. The effective R can then be
found. You can then make a graph of the results that can be used in
designs
before anything is built.
>>
The Q factor is important because it it should be optimum if the spark
gap
is to operate properly. Too great a Q factor will interfere with the
spark
duration. I do not know of anyone who has ever made these tests.

The true nature of the Q factor can be elusive as I show in the Tesla
Coil
Notebook. The Q factor is tied to the log decrement by a famous constant
ratio (pi). The Q factor is also equal to 1.364 times the number of
cycles
to reduce to 10% amplitude. It is obvious that this has an effect on the
spark gap firing. If this was better understook coilers would have less
problems with their spark gaps.

John Couture

------------------------------------------------------------
Hi John, All,
>>Have you ever tried the formula Q = sqrt( L / C ) / R.
>>
>>Barry
>>
>> ----------------------------------------------------
>
>  Barry -
>
>  I have never seen the above Q factor equation before. What would you
>use
>for R? In the usual equation   Q = X/R   where X = 6.283 F L  the R is
>an
>effective resistance, not DC or AC.  The only way to find the R is to
>use a
>value for a typical Q and X and solve the equation for R. The Q factor
>like
>the K factor is easily found by tests after the TC is built, however, at
>the
>design stage these two factors are difficult to determine.
>
>The basic nature of Q for Tesla coils has to do with the energy stored
>in
>the field of the coil and the energy dissipated. The Q factor also
>involves
>resonant rise in voltage and other electrical effects. It is also
>interesting that Q involves other phenonmena such as in the swing of a
>pendulum and in horological science ( measuring time).
>
>  John Couture
>
>

```