Linear and non-linear devices

Figure 2.6 shows a circuit in which current I can be varied by the variable resistor R2. For various settings of R2, the current flowing in resistor R1, displayed on
the ammeter, and the p.d. across R1, displayed on the voltmeter, are noted and a graph is plotted of p.d. against current. The result is shown in Fig. 2.7(a) where the
straight line graph passing through the origin indicates that current is directly proportional to the p.d. Since the gradient, i.e. (p.d.)/(current) is constant, resistance R1 is constant. A resistor is thus an example of a linear device.

If the resistor R1 in Fig. 2.6 is replaced by a component such as a lamp then the graph shown in Fig. 2.7(b) results when values of p.d. are noted for various current
readings. Since the gradient is changing, the lamp is an example of a non-linear device.