Voltage/Length (fwd)

From:  Malcolm Watts [SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent:  Tuesday, January 27, 1998 3:35 PM
To:  tesla-at-pupman-dot-com
Subject:  Re: Voltage/Length (fwd)

In reply:

> From:  John H. Couture [SMTP:couturejh-at-worldnet.att-dot-net]
> Sent:  Monday, January 26, 1998 2:44 PM
> To:  Tesla List
> Subject:  Re: Voltage/Length (fwd)
>   Malcolm -
>   You may be right. Let's get back to reality.  How would you measure the
> "no- infinity" impedance of the pick up ball in oil or air?

To measure the effect of a probe, a second one is needed to monitor 
the probe effects vs no-probe effects. Even this is subject to 
problems as each probe distorts the E-field. One approach is to 
position the probes on opposite sides of the HV terminal to shield 
the probes from each other.

>  What percentage
> of secondary voltage drop do you think it would cause on a Tesla coil ?

That entirely depends on the degree of loading caused by a particular 
probe. That is clearly an open question, given a lack of specifics. 

>  I would like to give the calculations a try. Have you ever tested for the
> TC secondary voltage and made the necessary calculations ? 

The closest I have come is to measure single shot attached spark 
lengths (impulse only) as D.C. Cox has done. I have compared those 
with measurements from other impulse sources (Marx Banks) whose 
theoretical outputs can be made to match real outputs very closely 
in practice. The 3'/MV figure is a guide I use although I recognize 
that extrapolating it in a linear fashion is dodgy at best. Pulse 
duration times will affect that figure. However, it is better than 
using a DC source that can hold up for a very long time (lightning is 
a prime example). For most TCs, using this guide to a couple of MV 
seems reasonable as most TCs would come in well under this figure, if 
our models and multiple attacks on the problem show consistency as 
they appear to do. As Greg states well, you *know* you are off if 
your measurements give a figure exceeding theoretical limits that are 
*proven* to be real. This brings us into the realm of model validity 
which is outside the scope of this post.

    As for using a divider or scope probe to measure the output 
directly, I have to acknowledge that I have not the components off 
the shelf to do the job. If I won a lottery and didn't have to bother 
about making a living, you can bet I would have the time to attack 
the problem from a number of angles with gusto.

> How do you do the
> calculations?

What calculations? Calculations are trivial if one has a formula but 
having a formula is dependent on formulating for a particular piece 
of hardware.

> In another post I  mentioned that even the 1000 meg divider I
> tried drops the sec voltage too much. However, Jim Monte's extrapolation
> test is a good possibility. What do you think?

I was merely trying to point out that terms are being bandied about 
that have no basis in reality. It would appear from what people have 
said that Tuve et al's work is flawed and therefore useless. One can 
*never* be exact in measurement. This is the number one rule of 
labaoratory technique. However, one can quantify errors as a 
percentage and thereby decide whether one's measurements are accurate 
enough to be considered usefully indicative for some particular 
purpose. Personally, I would accept a quanitifiable error of 1% in 
measuring TC output voltage as being close enough to be useful in 
choosing between models. Jim's technique is a good one in my opinion. 
It gets asymptotically closer to the real answer. It will never get 
there. Additionally, it must agree within the stated margin of error 
with at least one other technique (and preferably more) that is/are 
significantly different (e.g. one technique may distort the e-field 
more than another).