# Re: Number of turns for a secondary?

```In a message dated 1/5/99 2:43:54 AM Mountain Standard Time, tesla-at-pupman-dot-com
writes:

>
>  What is the wheeler equation??
>
>  mark
>
It is a formula for calculating the inductance of a coil.  It is convenient(at
least for us Americans) as all the dimensions are in inches.  The result is in
microhenries.

For a single layer solenoid:

L = r^2 * N^2 / (9*r + 10*b)
where r is the radius, N is the number of turns, and b is the length of the
coil(not to be confused with the length of the wire).  The accuracy is stated
to be within 1% as long as b > 0.8*r.

For a flat spiral coil:

L = a^2 * N^2/ (8*a + 11*w)
where a is the average radius, and w is the difference between the outer
radius and the inner radius.  The accuracy of this equation is within 5% if
2*a> w > 0.2*a.

The text also includes a disclaimer for the accuracy that states if the coil
has very few turn or the distributed capacitance is appreciable, then these
equations may be less accurate.
Hope this helps.
Mike

```