Conductors are ohmic if they have a constant resistance given that the physical conditions, such as temperature, are constant.
A filament bulb and thermistor are considered to be non-ohmic because they have a varying resistance. However, they only vary their resistance when they heat up, so aren't they technically ohmic?
Best Answer
In the case of the filament bulb, the test that is commonly used to produce the curvature in it's IV graph does not regulate the temperature of the device. This is technically an improper test for ohmic behaviour as Ohm's law states that the current is proportional to the voltage with all other physical conditions controlled (the main one being temperature). If you regulated the temperature, as a metal, the filament would probably produce ohmic behaviour and a constant resistance.
Similar logic would apply for a thermistor.
A truly non-Ohmic device would be a diode which does not let current flow when a negative voltage is applied.
Another important thing to note is that a real IV characteristics curve for an ohmic conductor where it's temperature is not controlled would not be perfectly linear. Heating effects of the current would cause changes in its resistivity. It is only when the temperature is controlled that you would get a linear graph.