The resistance of a tungsten bulb varies significantly depending on whether it is turned off or on.
Resistance When Off (Cold Resistance)
When a tungsten bulb is turned off, the filament is at room temperature. At this lower temperature, the resistance is significantly lower. For a typical 60-watt incandescent bulb, the cold resistance is approximately 24 ohms.
Resistance When On (Hot Resistance)
When the bulb is turned on and the filament heats up to its operating temperature (typically very high, around 2500°C), the resistance increases dramatically. This is due to the temperature coefficient of resistance of tungsten. For a standard 60-watt bulb operating on a 120-volt power source, the resistance is around 240 ohms.
Why the Difference Matters
The large difference in resistance between the cold and hot states explains why incandescent bulbs are more likely to burn out when they are first switched on. The initial surge of current, due to the low cold resistance, puts a significant strain on the filament.
Summary
| State | Resistance (approximate) |
|---|---|
| Off (Cold) | 24 ohms |
| On (Hot) | 240 ohms |
The resistance of a tungsten bulb is highly temperature-dependent; it's much lower when cold and increases substantially as the filament heats up to operating temperature.
