Re: Electrical Properties of Brass

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "K. C. Herrick by way of Terry Fritz <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Terry, I've always wondered about this reasoning.  Since the oxide layer
has high resistance, would not there be low RF current level in it?
Would that current, then, not have a much-reduced effect in establishing
the RF current path?  And would not the main RF current, then, seek a
path somewhat below the oxide layer, thus entering the region of
relatively lower resistance within the pure aluminum?

Ken Herrick

On Sat, 19 Apr 2003 09:20:53 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 >
 > Hi,
 >
 > "I" would never use aluminum.  Aluminum forms thick (skin depth
 > thick)
 > resistive oxide layers.  These are terrible for RF currents.
 >
 > The resistivity for pure Al is 2.7e-8 so at first it looks like a
 > good
 > choice.  But a 10 mil layer of fluffy white AlO2 and it goes bad
 > fast...  Since pure aluminum oxidizes in minutes and can reach 5 mil
 > in a
 > day easily, the RF resistivity can be just about anything but is
 > generally
 > bad.  Other metals form oxide layers too but they are far thinner
 > than
 > those of aluminum.
 >
 > Many people do use aluminum, but I would avoid it.
 >
 > Cheers,
 >
 >          Terry
 >
 > At 05:03 AM 4/19/2003 -0700, you wrote:
 > >Terry,
 > >
 > >That's really interesting! Can you do a similar
 > >analysis of aluminum roof flashing? I've often thought
 > >about cutting it into 1" ribbon and using it for
 > >primary coil stock.
 > >
 > >Regards,
 > >
 > >--- Tesla list <tesla-at-pupman-dot-com> wrote:
 > > > Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>
 > > >
 > > > Hi Jeremy,
 > > >
 > > > Resistance = Resistivity X Length / Area
 > > >
 > > > The numbers I have here are all in meters.
 > > >
 > > > Copper = 1.7 x 10E-8  Ohm-Meters
 > > > Brass = 6.39E-8 Ohm-Meter
 > > >
 > > >
 > >http://www.matweb-dot-com/search/SpecificMaterial.asp?bassnum=MBRASE
 > > >
 > > > So brass is about 4X more resistive than copper at
 > > > DC.
 > > >
 > > > Skin Depth = 1 / SQRT(pi x f x Resistivity x
 > > > Permeability)
 > > >
 > > > Permeability = pi x 4e-7  H/m  X  Ur
 > > >
 > > > Ur copper = 1
 > > > Ur brass = 1.05
 > > >
 > > >
 >
 >http://www.npl.co.uk/electromagnetic/dclf/magnetics/lowpermeabilitylc.ht
ml
 > > >
 > > > So the skin depth is about 1/2 that of copper since
 > > > the resistivity is 4X.
 > > >
 > > > The skin depth of copper is 2.602 / SQRT(F) inches.
 > > > At 154kHz that is
 > > > 0.00663 inches.  So brass is roughly 0.0033 inches.
 > > > Your 16mil brass is
 > > > perfect.  At 2 inches wide, it is about the
 > > > equivalent of 0.159 inch copper
 > > > tubing.
 > > >
 > > > So I will say your brass strips will be about 50%
 > > > more lossy than 1/4 inch
 > > > copper tubing.  Lots of "if, ands, ors, and buts"
 > > > too all this... but the
 > > > brass will work "ok" for a 15/120 NST system.  You
 > > > may loose about 2 to 4
 > > > inches as compared to 1/2 inch copper tubing or
 > > > something but probably not
 > > > a big deal.
 > > >
 > > > Cheers,
 > > >
 > > >          Terry
 > > >
 > > >
 > > >
 > > > At 04:28 PM 4/18/2003 -0700, you wrote:
 > > > >Hi!
 > > > >
 > > > >I was wondering if anyone has any experience with
 > > > >building primaries out of brass instead of copper?
 > > > >I want to use brass since I'm worried about copper
 > > > >oxidizing over time.
 > > > >
 > > > >I have a design in mind that is going to use
 > > > >brass strips for all connections in the tank
 > > > >circuit. (Including the primary)
 > > > >
 > > > >The brass strips I have in mind are approximately
 > > > >.016" thick by 2" wide.
 > > > >
 > > > > >From calculations I have done with JavaTC,
 > > > >the tank current at gap conduction is going
 > > > >to be approximately 800 Amps.
 > > > >
 > > > >(15KV/120mA NST .03 uf LTR primary cap)
 > > > >(154kHz resonant frequency)
 > > > >
 > > > >I am concerned that the brass strips I've got
 > > > >are too thin to handle this current load.
 > > > >Unfortunately, anything thicker is tough to work
 > > > >with and bend into the proper shapes I require.
 > > > >(Doubling up the thin strips is an option but
 > > > >I don't want to do it if i don't have to.)
 > > > >
 > > > >On the otherhand, since the tank current will be
 > > > >oscillating at RF frequencies, skin-effect tells me
 > > > >that the thickness isn't as much of an issue as the
 > > > >actual surface area.
 > > > >
 > > > >I guess the real question is, how far does a charge
 > > > >of 800A penetrate into brass at 154kHz ?
 > > >
 > > >
 > >
 > >
 > >=====
 > >Gregory R. Hunter
 > >
 > >http://hot-streamer-dot-com/greg
 > >
 > >