Linear and non-linear devices

The figure 2.3 shown below is showing a circuit. In this circuit, current I can be varied by making use of the variable resistor R2. For different setting conditions of R2, the current flowing in the resistor R1, displayed onthe ammeter, and the potential difference "p.d"across R1, displayed on the voltmeter, are noted and a graph is plotted of p.d. against the current. Diagram 2.4(a) shows the results where the straight line graph passing through the origin indicates that current is directly proportional to the p.d. Since the gradient, that is, p.d./current, is constant, resistance R1 is constant. A resistor, therefore, is a good example of a linear device.

We got the Figure 2.4(b), shown above, when values of p.d. are noted for number of different current readings. Since the gradient is changing, the lamp is an example of a non-linear device.