A filament lamp is initially at room temperature and is then connected across a variable d.c. supply so that the current through it is gradually increased.
(i) Describe qualitatively how the resistance of the filament changes as the current increases.
(ii) Sketch the expected I–V characteristic for the filament lamp and mark clearly the region over which resistance rises rapidly.
(iii) Explain why the filament lamp does not obey Ohm’s law even though it is made of a metallic conductor.