Not all electrical devices follow Ohm’s Law. These exceptions are referred to as non-ohmic devices. Recall that Ohm’s Law states that current flowing through a device is directly proportional to the voltage across its end, provided that temperature remains constant: $$ \begin{equation}\begin{aligned} V&\propto I\\ V&=IR\\ \end{aligned}\end{equation} $$
This means that if we observe the I-V characteristics (from the graph of current plotted against voltage), we will see for ohmic devices that the gradient will be constant. This constant gradient indicates that resistance remains the same. Gradient will change for non-ohmic devices - the resistance varies with the voltage applied to the device.
Ohmic devices will have a straight line for their I-V characteristics and non-ohmic devices will exhibit a curve.
I-V characteristics for a metallic conductor
Metallic conductors tend to be ohmic in nature:
I-V characteristics for a semi-conductor diode
The graph of current versus voltage for a semi-conductor starts at zero gradient then shows a quickly increasing gradient:
I-V characteristics for a filament lamp
A filament lamp’s I-V curve resembles a cubic function:
