In spite of all the discussion of conductivity and superconductivity in the last section, we really didn’t touch on how things conduct. Let's look at a very simple explanation to start with: the valence electrons in the electron sea are free to move under an electric potential.

If you understand that skip the next two paragraphs.

For the rest of us, think of electric potential as electric push-and-pull. Electrons have a negative charge. Put them in a region with a positive (attracting) charge at one end and a negative (repelling) charge at the other end, and negatively charged particles that are free to move will - toward the positive charge. In the lattice, the positively charged ions are not free to move - so the whole of the copper wire stays put when you flick the light switch (Whew!) The electrons that are still attached to a nucleus are also fixed. But the electrons in the electron sea are not attracted to a specific nucleus, so as long as another charge is just behind to replace it, a delocalised electron is free to follow its calling, so to speak.

In a battery electrons are supplied at one end (by one pole) and absorbed at the other, so the electrons keep on streaming through as long as the circuit is complete. The battery goes flat when the electric push-pull (potential) gets too weak to energise the electrons.