Tungsten is used in bulbs but not in iron boxes due to its unique properties and characteristics. Let's explore the reasons behind this difference.

1. Melting Point:
Tungsten has an extremely high melting point of around 3,422 degrees Celsius, making it ideal for use in bulbs. When an electric current passes through the filament of a bulb, it heats up and emits light. The high melting point of tungsten allows the filament to reach extremely high temperatures without melting, ensuring the longevity of the bulb.

2. High Resistance:
Tungsten has a high electrical resistance, which means it resists the flow of electric current. This property is crucial in bulbs as it allows the filament to heat up efficiently without excessive energy loss. The high resistance of tungsten helps to convert a significant amount of electrical energy into light energy, making bulbs more energy-efficient.

3. Ductility:
Tungsten is highly ductile, which means it can be drawn into thin wires without breaking. This property is essential for creating the filament in bulbs. The thin tungsten wire is coiled to increase the surface area, allowing for more efficient heat generation and light emission. In contrast, iron is not as ductile and would not be able to withstand the high temperatures required in a bulb.

4. Corrosion Resistance:
Tungsten is highly resistant to corrosion, making it suitable for use in bulbs where the filament is exposed to high temperatures and the surrounding environment. Iron, on the other hand, is prone to rusting and corrosion when exposed to moisture and heat, making it unsuitable for use in a bulb.

5. Heat Conductivity:
Tungsten has relatively low thermal conductivity, meaning it does not easily transfer heat to its surroundings. This characteristic is beneficial for bulbs as it allows the filament to retain heat and reach the high temperatures necessary for light emission. In contrast, iron is a good conductor of heat and would dissipate heat quickly, making it inefficient for bulb applications.

In conclusion, tungsten is used in bulbs due to its high melting point, high resistance, ductility, corrosion resistance, and low thermal conductivity. These properties make tungsten an ideal material for creating efficient and long-lasting light sources. Iron, on the other hand, lacks these properties, making it unsuitable for bulb applications.