>>13498045the key is to understand the difference between drift current and diffusion current.
drift current is current caused by an electric field (i.e. voltage), what you normally think of as current.
to understand diffusion current, imagine a sealed box with a divider in the middle, with air on one side of the divider and vacuum on the other. if you remove the divider, the air quickly rushes into the other side until both sides have the same amount of air. no force actually pushed the air into the other side, but the air molecules are naturally bouncing around inside the box, and once the divider is removed, some molecules just so happen to move to the other side. once both sides have the same amount of air, then the amount of molecules moving from side A to side B is the same as the amount moving from B and A, and thus there's no "wind" in the box.
now, imagine the conductors gold and copper, and assume both are electrically neutral at first. even though there's no net charge present on either, the gold will have more electrons, since the gold has more protons. when you put the gold and copper together, the imbalance of mobile charges (electrons) between the two will cause a net current from the more populated side (gold) to the less populated side (copper). this causes the gold to become more positve and the copper negative. this means that there will be an electric field that steady grows pointing from the gold to the copper. as the field grows, it pushes the electrons back to the gold side (drift current). once the drift current and the diffusion current are equal, the system is steady and no electrons are flowing, but the electric field is still present, and thus a voltage exists across the plates. this steady-state voltage is a function of the amount of free electrons in the conductors, which is a function of the temperature of the conductors. this is how a thermo-couple works.
