OK now imagine that lil thing as say 100 sqcm of metal clamped together and heated to the many hudreds of degrees that would come from a fire (yes we still have a kitchen fireplace for the winter). as I recal from chemistry lessons on battery cells the voltage value is defined by the metal mainly and so by finding the one that is most - and most + u get a p.d. obviously this is also going to depend on the temperature in my application as well and so will the current. I pressume a similar logic is applicable in this case taking into accout the physical size of the juction and the temperature. the voltage had something to do with the chemical composition of the metals used