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# Re: Wheeler (Number of turns for a secondary?)

```Tesla List wrote:
>
> Original Poster: Hollmike-at-aol-dot-com
>
> In a message dated 1/7/99 3:19:01 AM Mountain Standard Time,
tesla-at-pupman-dot-com
> writes:
>
> > >
> >  > L = r^2 * N^2 / (9*r + 10*b)
> >
> >  This is also a translation to inches of the formula in metric units:
> >  L = 4*pi*1e-7*pi*r*r*N*N/(b+0.9*r)
> >
> >  This is simply the formula for the inductance of a long coil (r<<b)
> >  with an approximate correction factor 0.9*r added to the length b.
> >
> >  Antonio Carlos M. de Queiroz
> >
> >
> Antonio,
>     That is pretty much the classical formula for long coils, but most TC's
> are not that geometry.
>
>     If one wants to use metric,  The wheeler formula can be altered quite
> easily.  In fact you can simply put in the r and b values in centimeters and
> just divide the whole expression by 2.54.
>
> L = r^2*N^2/(2.54*(9*r + 10*b)), where now the dimensions are in
centimeters.
> The inductance is still in microhenries.
> Mike(slowly converting to metric, inch by inch )

Wheeler's approximation works within very acceptable accuracy for any
coil geometry you will encounter in TC work.  I have much more accurate
expressions here, in case anyone wants one.  The limit on your accuracy
may well be the accuracy with which you really know the coil dimensions.

Ed

```