Re: Aluminium aka Aluminum Wire (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 02 Oct 2007 06:34:17 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Aluminium aka Aluminum Wire (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Mon, 1 Oct 2007 22:53:01 -0700
> From: Ray von Postel <vonpostel@xxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: Re: Aluminium aka Aluminum Wire (fwd)
> 
> I would think that if the metal used in the wire to wind a coil made 
> any difference to the INDUCTANCE, that it would be a factor in 
> equations used to calculate INDUCTANCE.  I don't find it in equations 
> used by NBS/NIST when computing the inductance of standard inductors.  
> Does any one say they have missed something?
> 
> Two coils having IDENTICAL geometry, one wound with aluminum and the 
> other copper will have the same INDUCTANCE. However, the losses in the 
> coil wound with aluminum will be higher because it will have a higher 
> resistance.


Yes and no.

The inductance is due to the interaction of the magnetic field of one 
part of the inductor with other parts of the inductor, and, obviously, 
the location where current flows affects that magnetic field.  Skin 
effect results in AC currents flowing more towards the surface than in 
the core of the conductor. If the current distribution is different, 
then conceivably the inductance will be different.

Here's a gedanken example:

Consider a two turn coil, with turn A and turn B beside each other 
spaced at some distance S. First, assume that the wire is infinitely 
thin.  The magnetic field that cuts across turn B due to the current 
flowing in turn A is proportional to I/S.  Now, split turn A into two 
parallel wires, spaced "d" apart, each carrying half the current (I/2). 
   The field at turn B is now:
0.5*I/(S+d/2) + 0.5*I / (S-d/2) = I * 2*S/(S^2-(d/2)^2)  which is NOT I/S.

Skin effect is just like taking the current and splitting it into a 
whole raft of little parallel wires arranged in a circle.


Now... for good conductors, the skin depth doesn't change much, so the 
effect on the inductance will be small (square root of conductivity)... 
probably much less than the accuracy of the usual inductance table/formulas


> 
> Similarly, capacitance is not dependent upon the material of the 
> conductor and only upon the geometry.  This assumes the dialectic 
> remains the same in every way.

Depends on charge distribution, but in real devices of finite size, 
there's always a parasitic L that's in series, and, that L varies with 
the conductivity because of the change in current distribution.  Small 
effect, though.
> 
> 
> Cooper-weld wire was developed as an engineering compromise.  Antennas 
> for low frequencies, such as the Beverage, flat top and multi-curtain 
> rhombic become physically large.  The use of copper-weld is a 
> compromise between and among cost, strength, and r. f. resistance.

Copper clad steel has been around longer than antennas, I suspect.  The 
primary use was for telegraph wires.