Resistance of an electric lamp and other conductors
The ammeter-voltmeter method is an excellent one for measuring the resistance of an electric lamp when run at different voltages. The circuit shown in Fig. 40.4 (a) is used in which the single cell is replaced by any convenient 6 V d.c. source and the resistor R changed for a 6 V electric lamp. The results show that the resistance of the lamp filament increases with temperature. A mean value should therefore not be worked out. Instead, a graph of resistance against voltage, or else voltage against current, is more instructive.
Fig. 40.5 shows typical voltage-current curves for various conductors. Look again at what we said earlier about the gradient of such curves, and also page 26.