This apparatus was used by Ohm. Current flowing through the metal bar in the center cylinder deflects a magnetized needle suspended above it. The deflection angle is proportional to the current. The source of electric potential is a thermocouple (discovered by Seebeck in 1821). The ends of the thermocouple are heated by steam and cooled by ice-water in the small containers on the tripods. The use of a thermocouple made the measurement possible; other sources of potential available in the 1820's were too unreliable.