Re: [Phys-l] current vector

[Bernard Cleyet <bernardcleyet@redshift.com>]

This is an instructive problem, not simple *, -- because AgS is 
conductive, but poorly compared to pure Ag, it "absorb and heats 
(becomes heated?)";  at the same time the skin depth, I suspect, 
increases to the more highly conductive pure Ag.  I didn't find the 
conductivity or typical depth (is AgS protective as is Al2O3? or more 
like FexOy?) of AgS, so one could compare fresh a microwave component 
vs. an aged one.

* like grin or a cap. w/ two dielectrics.

bc, who has both Ag, Al, and gold microwave components.

Brian Whatcott wrote:

> At 04:44 PM 2/20/2006, John D., you wrote:
>  
>
> > //
> > For sure at high frequencies, the current is strongly concentrated
> > at the surface.  This is the usual "skin depth" calculation.
> >
> > A lot of microwave plumbing is gold-plated.  Indeed some of it
> > is gold-plated brass, or gold-plated steel ... and it could be
> > gold-plated plastic and work just as well, because the skin
> > depth is so small that the waves never make it past the plated
> > layer.
> >    
>
> Conductors carrying current at very high frequencies
>  (VHF, UHF, SHF on up) have often been plated to take advantage of
>  surface effect. The material used in the past has been electrolytic
>  silver. Reviewing the comparable resistivities shows why:
> [values vary from data source to source]
>
> aluminum    2.65            X 10E-8 ohm.m (a  weaselly unit for uOhm.cm)
> brass       ~8
> cobalt        6
> copper       1.7
> gold           2.4
> iron          10
> magnesium  4
> phosphor bronze 7
> platinum    11
> silver         1.6
> sodium       4.5
> tin            11
> zinc          5.9
> (Science Data Book Tennent,  Oliver&Boyd O.U.)
>
> Silver has the modest advantage - depending on the alloy,
>  and on its surface state.   But silver tarnishes - the tarnish
>  is said to be conductive still, but its conductivity drops and
> demands (or demanded) replating.
> Gold has the virtue of constancy in corrosive conditions
> despite conductivity not much better than pure aluminum.
> A (thin) gold flash on silver is a good surface.
>
>
> Brian Whatcott    Altus OK    Eureka! 
>
>
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