Georg Simon Ohm, German physicist, c 1820.

Georg Simon Ohm (1789-1854) discovered the law governing the relationships between voltage resistance and electric currents. His 'Ohm's Law' states that the relationship of current through most materials is directly proportional to the potential difference applied acros the material (V=IR). This was published in his book 'Die galvanische Kette, mathematische bearbeitet' in 1827, in which he gave his theory of electricity. The SI unit of electrical resistivity, the Ohm, is named after him.