A resistor is an electrical device that resists current, and the difference between ohmic and non-ohmic resistors relates to how the resistor reacts to different types of current that pass through it. In an ohmic resistor, the resistance provided is the same regardless of the type of current that passes through the device. In a non-ohmic resistor, the resistance changes depending on the type of current passing through it.
The designation of ohmic versus non-ohmic involves an electrical law known as Ohm's Law, which was developed by Georg Ohm. Ohm's law basically says that the current in a circuit is proportional to the amount of voltage in the circuit. Because of this proportionality, when voltage and current are plotted on a graph, their relationship is linear. An ohmic resistor also has this linear relationship if its current and voltage are graphed, according to the physics Web page for Nayland College. Non-ohmic resistors, on the other hand, have an irregular graph that is not linear.
It is important to note that the distinction between ohmic and non-ohmic resistors can be made only when the conditions of the resistor are the constant. If conditions are not constant, then the distinction cannot be made between ohmic and non-ohmic resistors.