How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 1253

How does an acetylene miner's lamp work? How does a propane gas lamp work? Why do gas lamps need a mantle and what is the mantle made of? — DK, Washington, DC
An acetylene miner's lamp produces acetylene gas through the reaction of solid calcium carbide with water. An ingenious system allows the production of gas to self-regulate—the gas pressure normally keeps the water away from the calcium carbide so that gas is only generated when the lamp runs short on gas. In contrast, a propane lamp obtains its gas from pressurized liquid propane. Whenever the propane lamp runs short on gas, the falling gas pressure allows more liquid propane to evaporate.

Only the propane lamp needs a mantle to produce bright light. That's because the hot gas molecules that are produced by propane combustion aren't very good at radiating their thermal energy as visible light. The mantle extracts thermal energy from the passing gas molecules and becomes incandescent—it converts much of its thermal energy into thermal radiation, including visible light. Mantles are actually delicate ceramic structures consisting of metal oxides, including thorium oxide. Thorium is a naturally occurring radioactive element, similar to uranium, and lamp mantles are one of the few unregulated uses of thorium.

The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.

In contrast, the acetylene miner's lamp works pretty well without a mantle. I think that's because the flame contains lots of tiny carbon particles that act as the mantle and emit an adequate spectrum of yellow thermal radiation. Many of these particles then go on to become soot. A candle flame emits yellow light in the same manner.

One last feature of a properly constructed miner's lamp, a safety lamp, is that it can't ignite gases around it even if those gases are present in explosive concentrations. That's because the lamp's flame is surrounded by a fine metal mesh. This mesh draws heat out of any gas within its holes and thus prevents the flame inside the mesh from igniting any gas outside the mesh.

         

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