It was the German Thomas Johann Seebeck (1770-1831) who in 1821 showed for the first time that by heating or cooling one of the soldered joints, the magnetic needle tended to orient itself perpendicularly to the plate, thus indicating the passage of an electrical current; the direction of rotation of the needle always indicates that in the copper the current goes from the warmest to the coolest joint.
He thus demonstrated that by creating a difference in temperature between the two soldered joints a difference in potential is created between them (the Seebeck effect).
In this model, since the earth's magnetic field tends to make the two needles rotate in opposite directions and the current makes them rotate in the same direction, the astatic system, besides eliminating the action of the earth's magnetic field, also doubles the sensitivity of the instrument.
The one with two needles, designed in 1821, is one of the systems proposed and used by André Marie Ampère (1775-1836), to eliminate the disturbing action of the earth's magnetic field in experiments in which the purpose is to reveal the deviation that a magnetic needle undergoes through interaction with an electrical current.