Contact potential between metals
Every junction between dissimilar metals has a potential associated. In your example, you have three such junctions: Cu-Al, Al-Au and Au-Cu. If they're all at the same temperature, these potentials will all cancel out. This is required by simple thermodynamics &mdash; if they didn't cancel out, you'd be able to extract arbitrary amounts of energy from the system.

But if they're not all at the same temperature, you will be able to measure a voltage, and you'll be able to extract electrical energy from the heat flow through the system. This is the basis of the Seebeck effect.