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# Re: Medhurst now empirical formulas

```In a message dated 7/12/99 3:27:33 AM Central Daylight Time, tesla-at-pupman-dot-com
writes:

<< Way back in the 1930's  >>

Bryan, Ed, Terry--all,

The 1930's were "only yesterday." :-((

Thanks for the Medhurst info and the QuickBasic file. This latest entry into
the stupid
question contest has helped me because outside of the Tesla List I have never
seen the Wheeler and Medhurst names attached to the to the various empirical
equations for the intrinsic inductance and capacitance of a solenoid.

Terry writes that the other empirical equations are based on the Medhurst
data. In Terman's Radio Engineering, 2nd ed., he shows a graph for
determining the self inductance of a solenoid based on the ratio of H/D.
Basically Wheeler, Terman graphs an
empirical constant, F, vs  the ratio of H / D for a range of 0.1 to 10.1 to
solve the empirical
equation Lself = F x diam x N squared.

The Medhurst data can be found at
www.peakpeak-dot-com/~terryf/tesla/misc/medhurst.jpg
Thanks Terry, one giant leap :-)). I graphed k vs the ratio of H / D for a
range of 1.0 to 5.0
to solve the empirical equation for the intrinsic capacity of a solenoid:
Cself = k x diam.
I find the graphs of greater value than the equations; can anyone explain how
far one
can safely extrapolate before strange things happen?

In the appendix of Terman's second edition, there is an excellent discussion
of various
empirical formulas for the intrinsic capacitance and inductance for all forms
of coils including the toroid, and for many of the capacitance problems
including
transmission lines and solenoids.

Bryan writes: "Has anyone found the physical basis for the data?"

Terman is speaking specifically about the inductance equations but I think
the statements
also apply to the empirical capacitance equations as well. Terman writes:

"Skilled mathematicians have derived formulas that give the inductance of all
the
commonly used types of coils with non-magnetic cores in terms of the coil
dimensions.
These formulas are usually both complicated and hard to derive because of the
difficulty
encountered in calculating the magnetic flux produced by the current flowing
in the coil.
In order to make such formulas of practical value, they are always simplified
by the use of coefficients."

Well, that's good enuff for me, first things first, it's pool time!! :-))

The Tesla List is the greatest.
Ralph Zekelman

```