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Re: [Phys-l] current vector



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!