The "it" in the title refers to what is now known as Ohm's law. Georg Simon Ohm (1789-1854) lived at a time when there were no calibrated indicators for electric current. There was no volt or amp; these were established much later by the 1881 International Electrical Congress. The resources available to Ohm were: 1) the discovery of Oersted, who in 1520 showed that a magnetic field surrounded a wire carrying electric current; 2) the electrochemical cell, described by Volta in 1800; and 3) the thermoelectric effect, discovered by Seebeck in 1822. How Ohm discovered his law with these varied and limited resources is the subject of this article.