Re: what property of metals makes them useful for overhead power cables?

This is a quote:

"In a certain class of substances, particularly metals, the outer orbital electrons of an atom appear to be loosely bound to the atom. One or more of them may become detached from an atom and roam about with random motion. These are called Free Electrons, and materials which contain large numbers of free electrons are called conductors."

Using aluminium in place of copper is an economic choice. Actually, silver is a much better conductor of electricity than either aluminium or cooper, but it costs far too much to use for that purpose. Alumininium is cheaper than copper. For the same size wire, it also has more resistance to current flow than does copper. Therefore, a larger aluminium wire must be used, and since space is not a requirement for high voltage tranmission lines, it is the popular choice.

Copper is still used for most other applications because aluminium will corrode, very slightly, and that corrosion is a very good insulator. It is subject to what is called "contact resistance" where it connects to other devices, and that resistance may be so high it may cause the current flow to stop, or the wire to melt out of the connector.

Copper doesn't do that, so most installations of aluminium wire have had that material removed and replaced with copper whose dark coloured corrosive surface film is not an insulator.