Algodoo

The lopsided ring represents two wires of different alloys.
The left lasers represent a hot temperature.
The right lasers represent a low temperature.
The circles crudely represent electrons.
At the junction of two different metals, when heat is applied, electrons are free to move from the "thinner" wire to the "thicker" one, and vice versa.
In the real world, it isn't thinner or thicker wires, it's their conductivity, thicker being the less resistive and thinner being more resistive.
W to apply heat differential, this causes a voltage, causing the purple bars to spin.
Electrons now change color depending on energy level