The most recent mark scheme makes these points:
- For low voltages, the line is straight so current is proportional to voltage
- As the current increases, the filament wire heats up
- This increases the resistance
- Increasing the voltage will push more current through, but because the resistance has increased, the current will not go up as much for an equal increase in voltage
- The same thing happens if you put current through the bulb in the opposite direction so the characteristic is symmetrical.
