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Re: a frequency-variable resistor
At 16:11 11/05/99 -0600, Bert Hickman wrote:
... Snip ...
>In effect, we get hit with a double-whammy! The steel already starts out
>being 5.8 times more resistive than copper even for DC. Because of its
>higher permeability, RF current then is forced to flow through a much
>thinner "skin" of this poorly-conducting material (the skin depth of
>steel is only about 17% that of copper). The combination of these two
>effects means that a steel conductor will be 32 TIMES lossier than an
>identically-sized copper conductor at RF frequencies.
Not that any of you would doubt Bert for a moment, but I have actually been
there and seen this under real-time conditions.
When I made my first coil, for the primary I used a reel of copper plated
steel tubing (copper was only about 10 thou thick), as my father-in-law
worked for a tubing company and I got for nicks ;-)
Through the trials of getting better performance Malcolm suggested I change
this for solid copper tubing (both were 1/4"), and I have to confess 2
significant changes:
1. The S/S tubing used to get too hot to touch on longer runs, cf. the
copper remained perfectly cool (read energy not being lost to resistive
heating), and
2. The sparks were longer ........... which is what you really care about,
eh !!!
This isn't just one for the theory boys, it actually matters.
Now if only I could finish the other 50 projects and get back to some
coiling :-((
Getting withdrawal in the great southern land ........
Mark
http://www.cobweb-dot-com.au/~dkfinnis