Fixed resistors are designed to obey Ohm's law under normal changes of temperature when current flows. They are described as ohmic conductors. The resistor behaves in the same way regardless of the polarity.Most wires and metallic conductors are ohmic.Filament lamps do not obey Ohm's law and are non-ohmic conductors. They behave the same way regardless of polarity.As the potential difference increases, the current increases and more electrons flow through the wire every second.As the current increases there are more collisions per second with the positive metal ions so the electrons transfer more energy to the positive metal ions which vibrate more. As the temperature and vibrations increase there are more collisions so the resistance increases, so the current increases by less for each rise in p.d.The wire gets very hot so it glows.