Effect of temperature on current in carbon or tungsten filaments.

Effect of temperature on current in carbon or tungsten filaments. This demo is similar to D+55+2. A light bulb is substituted for the electric heating element. The thing to note here is the difference between the characteristics of a tungsten filament bulb, and an old carbon filament Edison bulb. When the knife switch is thrown, the initial resistance of a modern tungsten filament is low. There is a surge current at turn on. Then as the filament heats up (and the resistance rises), the current rapidly drops, (and the voltage drop across the bulb rises). The carbon filament has a higher initial resistance (no surge current), and the resistance drops more slowly as the filament heats up.