Re: Electrical Properties of Brass

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Mark,

At 11:57 AM 4/19/2003 -0500, you wrote:

>However, it is still difficult to make a good electrical connection with 
>the aluminum, since oxidation occurs in minutes.  Connections have to be 
>made air tight to prevent the oxidation process, which probably already 
>started before the connection was made air tight (work in an argon-rich 
>atmosphere, perhaps?).

In our case, the voltage is so high that it will blow through any 
resistance caused by connection oxidation.  But it is still a good idea to 
clean up any aluminum connection with sandpaper first and make sure the 
connections are airtight.  They sell a grease for this purpose at 
electrical supply places.


>So I, too, would agree with Terry's recomendation against using it.
>
>Once you've optimized your TC, you could coat a copper primary with a very 
>thin layer of polyurethane.  Then just scrape away the PU at the tap 
>point. This doesn't make future modifications/tweaks too easy, though.

I think painting it with clear poly is a great idea. Probably much easier 
than other alternatives.  Clamps and such may oxidize at the scratches if 
one moves them around but that will be a problem anyway.  Another 
alternative may be to spend another $12 and just replace the primary copper 
if it gets too bad.


>Happy Easter
>
>Mark Broker
>Chief Engineer, The Geek Group
>
>
>
>On Sat, 19 Apr 2003 09:20:53 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:
>
>>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.html
>>> >
>>> > 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
>>> >
>>> >
>>> >
>