When you increase the resistance of a filament bulb more work is done on the bulb which means its temperature increases. This causes the ionic lattice to vibrate with a greater amplitude (since it has more kinetic energy). The conduction electrons now encounter more collisions, hence the resistance goes up. What I was wondering is why this does NOT work for a fixed carbon resistor. If you increase the voltage across a fixed resistor it heats up (you can feel it) but the voltage and current remain in direct proportion. What is the reason for this? If a resistor is hot surely the atoms in the carbon resistor are vibrating more (as temperature is proportional to the average kinetic energy)? So how can the resistance not be affected?