graphical properties of non-ohmic resistances

This figure shows that the relation between the current i and v in case of vaccum tubes i.e., an electronic device
This figure shows the V-i characterstics of a P-N junction diode rectifier which shows that non-linear relation between the voltage and the current in positive direction and in the reverse direction. This figure shows increase in the current is not possible beyond zeroth value of the voltage, so that either in positive value of V or negative value of V, for large value of the voltage, there is increase in the current.
This figure shows that the V-i characterstics of thermistor. The detail study of thermistor we can see in electronics subject.