When Edison was testing filaments he looked for materials that did not melt easily when heated. He also discovered that he wanted materials that had a high resistance. Resistance is a measure of how well a wire prevents the passage of electricity through the wire. In order to increase the resistance of a metal wire, Edison created coiled filaments, so a longer wire could be used in the same space. (Longer wires have a higher resistance.)

Iron is a very hard metal, with a high melting point (1535¾C). It also has a high resistance, similar to that of platinum.