- To demonstrate an IV plot of a heated filament
The horizontal input of the oscilloscope is connected to display the lamp voltage (V), and the vertical input to display the lamp current. At any constant AC voltage, the slope of the I-V plot is the filament's conductivity at constant temperature (as the Joule heating is I squared R, there are 120 heating cycles per second and insufficient time for the filament to change temperature).
Notice that the slope is a straight line, indicating that Ohm's law is obeyed. Now notice how the slope changes when the AC voltage is increased or decreased, showing that the filament's resistance changes when the filament temperature changes. Hence, we see that Ohm's law is valid at a fixed filament temperature, but fails when the filament temperature changes.