On Friday, January 6, 2017 at 9:52:54 AM UTC-5, wrote:

SNIPPAGE

a) Wire conductivity, irrespective of material, is a function of surface area and gauge amongst other factors. So, if 14-gauge wire is then divided into strands, the final product with the most strands will have the most surface area, and so by some factor greater conductivity.

b) Using material conductivity hierarchy, the most conductive material wants to be on the surface. Aluminum is a much poorer conductor than copper. All other things being equal, aluminum coated copper will be a poorer conductor than copper-coated aluminum (or steel: c.f. CopperWeld).

c) Corrosion is a factor, but not much of one. Alumina (AKA Aluminum Oxide) is an excellent insulator, as well as being quite inert. As aluminum wire acquires an oxide coating very nearly immediately upon exposure to air, that will reduce conductivity on the immediate surface of the wire. But as that coating is only a very few nanometers thick, the effect is near-zero overall. Copper oxides and sulfates as well as various other 'salts' are variously conductive, but also can be rectifiers, so there will be some effect on overall conductivity in the presence of these materials. BUT, copper is not a very active metal, so oxidation under normal atmospheric and environmental conditions is low and slow. Similar to Aluminum, but for different reasons.
d) Electrolysis is always a factor when dissimilar metals are forced into a conductive connection in the presence of oxygen or other oxidizers. For an interesting experiment, take an ordinary 16D nail, wrap a bit of copper wire around it, and place it in a glass of ordinary tap water. In an adjacent glass place a similar nail, no copper. Then a bit of copper, no nail. Do the same with an aluminum wire bonded to a copper wire (mere wrapping may not insure sufficient connectivity for electrolysis to take place. Copper clad materials generally do not suffer as there is very limited surface area exposed.

Enjoy!

Peter Wieck
Melrose Park, PA