The Bimetallic Compass
Thomas Johann Seebeck and Thermoelectric Diffusion in Metals
A closed metallic ring formed by soldering a bar of bismuth to a bent strip of copper contains no acid electrolyte, no zinc-copper voltaic pile, and no permanent magnet. Yet when Estonian-born physicist Thomas Johann Seebeck held one of the two soldered bismuth-copper junctions in his warm hand at the Prussian Academy of Sciences in Berlin in August 1821 while leaving the opposite junction at room temperature, a magnetic compass needle suspended inside the metallic loop swung thirty degrees off north. Packing snow around the heated junction reversed the compass deflection toward the opposite side. Heating one junction with a spirit lamp drove the needle nearly perpendicular to the bars.
One year earlier, in 1820, Danish physicist Hans Christian Ørsted had demonstrated that an electric current flowing from an Alessandro Volta chemical battery of stacked zinc, copper, and acid-soaked cardboard disks deflected a nearby compass needle. Galvanism and electromagnetism were universally understood to require chemical reaction—the wet oxidation of metal plates by acid solutions—to pump electric charge around a circuit. Dry solid metals soldered together were regarded as passive conductors incapable of generating electromotive force on their own. Seeking to test whether metallic contact without chemical moisture could influence magnetism, Seebeck spent seven months between August 1821 and February 1822 testing rings made from twenty-eight different metals, alloys, and mineral ores across four reports to the Berlin Academy.
Seebeck discovered that all conductors could be ranked in a fixed directional series, with bismuth, nickel, and cobalt at one extreme and tellurium, antimony, and iron at the other. Heating one junction of any pair deflected the compass in a direction determined by their relative positions in the list, and by an angle proportional to the temperature difference between the hot and cold solder joints. Paradoxically, Seebeck rejected the conclusion that an electric current was flowing through his bimetallic rings. Convinced that thermal gradients directly magnetized metals without moving electric charge, he named the phenomenon thermomagnetism. He even argued that the temperature difference between the sun-warmed equatorial belt and the frozen polar caps turned the Earth's metallic crust into a giant thermomagnet.
It was Ørsted himself, examining Seebeck's bismuth-antimony circuits in Berlin during 1822 and 1823 alongside Jean-Baptiste Fourier, who proved that the heated junctions were driving a continuous electric current governed by Ampère's circuital law and renamed the effect thermoelectricity. Four years later, German physicist Georg Simon Ohm struggled to measure how wire length and cross-section affected electrical resistance because wet acid voltaic piles polarized and decayed from minute to minute. Advised by Johann Schweigger, Ohm replaced his chemical battery with a copper-bismuth Seebeck thermocouple whose two junctions were immersed in boiling water ($100^\circ\text{C}$) and melting ice ($0^\circ\text{C}$). Powered by that rock-steady thermoelectric voltage, Ohm discovered his linear law of electrical conduction ($I = V / R$) in 1827.
The physical engine inside Seebeck's dry metal ring could not be explained until twentieth-century quantum solid-state physics replaced classical fluid models of electricity with the Fermi-Dirac electron gas. Inside any metallic conductor, outer valence electrons occupy quantum energy levels up to the Fermi energy. When one end of a metal bar is heated while the other remains cold, the thermal distribution of electrons near the Fermi surface broadens at the hot end. High-energy electrons at the heated junction possess higher mean thermal velocities and different scattering rates against lattice vibrations than electrons at the cold end. Consequently, energetic electrons diffuse down the temperature gradient from the hot junction toward the cold junction.
In an isolated wire made of a single metal, that thermal diffusion quickly halts itself. As negative electrons pile up at the cold end and leave exposed positive metal ions behind at the hot end, they create an internal opposing electrostatic voltage—proportional to the material's Seebeck coefficient $S$ in microvolts per kelvin—that balances thermal diffusion. In a closed loop made of a single uniform metal, both arms connecting the hot and cold points develop the exact same opposing voltage, so the potentials cancel to zero around the ring. When two dissimilar metals such as bismuth ($S \approx -72\text{ }\mu\text{V/K}$) and copper ($S \approx +6.5\text{ }\mu\text{V/K}$) are soldered into a ring, their distinct electronic band structures near the Fermi level generate unequal diffusion voltages down the two arms, driving a continuous current around the circuit. In 1834, Parisian physicist Jean Charles Athanase Peltier demonstrated the exact thermodynamic inverse by driving an external battery current across a bismuth-antimony junction at constant room temperature, where electrons climbing the Fermi-level step absorbed lattice heat and froze a droplet of water resting on the solder joint.