The changing resistance of a filament bulb.

We test the resistance of the bulb, as we would test any resistance, in a circuit like this:

Circuit to measure the changing current of a filament bulb

Circuit to measure the changing current of a filament bulb

From the results of potential difference and current we plot a graph:

Current/voltage graph for a filament bulb

The filament of a filament lamp is metal, but the resistance does not remain constant as you can see from the graph line, which is curved. If it was straight then it would be obeying Ohm's Law, the resistance would be a single value. But the temperature does not remain constant, as the current rises the filament glows more, it gets hotter.

The graph of current against resistance is curved because  the PD (voltage) rises at a faster rate than the current. As the filament gets hot the metal atoms vibrate and the outer electrons of the atoms become more excited. The flow of electrons carrying the current is impeded by collisions with the excited electrons, therefore raising the resistance.