Students should be able to explain that, for some resistors, the value of R remains constant but that in others it can change as the current changes. • The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes. • The resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.