Carbon has the highest resistivity of all the elements—it’s resistivity is over 100 times greater than that of a metal such as platinum. To create the carbon filaments, Edison partially burned different materials until the material changed into carbon. Because carbon filaments could be very fragile, Edison used a looped filament, rather than a coiled filament. The filaments were shaped in molds before they were carbonized.

Edison tested many different types of paper filaments, including cardboard and writing paper. The filaments were from thin strips of paper of different widths.