Carbon Bulb vs. Tungsten Bulb
This graph shows the resistance of the filament in two types of light bulbs. The blue curve shows that a carbon filament decreases in resistance as the bulb heats up, and the pink curve shows that a tungsten filament bulb increases in resistance as it heats up.
Thus, carbon filament bulbs have a negative temperature coefficient and tungsten filament light bulbs have a positive temperature coefficient.
Tungsten is the filament material most commonly used in household light bulbs.
Incidentally the curve for the carbon filament bulb stops short at 90V because I don't want to damage it. It runs very, very hot!