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.

Unlike paper, the cotton thread already had a thin, wire-like shape, so it was easy to create carbonized cotton thread filaments.