# Re: New Inductance Formula

```Original poster: "Shaun Epp by way of Terry Fritz <twftesla-at-qwest-dot-net>" <scepp-at-mts-dot-net>

Hey dave,

Yup, way off.  The reason is that is the equation L = N^2*u*A/l  is for a
long solenoid, where length is 10x or more larger that radius, your example
was for a short fat coil, there are equation specially for this, but I can't
find them right now.  I tried both equation, this one and wheelers for a
solenoid using my tesla secondary as an example.

r = 2.125 "
l =  22 "
N = 1400 turns
convert valued to metric for the non wheeler equation and I get:

L = 40.3 mH from the above equation and,

L = 37.01mH from wheelers equation,  and the same from my metric version.

I see you included units again,  I think that Webers/amp is a much more
meaningful unit for electrical/magnetic  measurements that the base units
that you have been using.

All the best,

Shaun Epp,  goin' for another round 8 - )

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, May 04, 2002 1:36 PM
Subject: RE: New Inductance Formula

> Original poster: "David Thomson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>
>
> Hi Shaun,
>
> >And Dave, the reason that inductance formula is way off is because it is
> missing a "pi", see below:
> >> In the book Physics, by Edward R. McCliment, Univ of Iowa, the formula
> for
> >> self inductance is given as:
> >>
> >>         4pi * km * R^2 * N^2
> >> Lself = --------------------
> >>                  l
> >
> >        4 * pi^2 * 10^-7 * R^2 * N^2
> >Lself = ----------------------------
> >                     l
> >which is the expanded version of that physics text inductance formula for
a
> long selenoid.  I expanded u and area.
>
> You are correct that I missed the pi.  But the actual expanded formula is:
>
>         4 * pi^2 * 10^-7 *tesla * m * R^2 * N^2
> Lself = ---------------------------------------
>                      l * amp
>
> And this formula, while it gives the correct units, is 356% greater than
> Wheeler's when:
>
> R = .61m
> l = .305m
> N = 411.5
>
> Dave
>

```