Have you ever wondered why tungsten light bulbs develop a black deposit as they age? This deposit is metallic tungsten, the same stuff that the filament is made of. A lightbulb produces heat and light when an electric current passes through a resistance. In this case, the resistance is a tungsten filament. Tungsten is close to ideal for a filament because it has a very high melting point and does not evaporate easily. Nevertheless, small amounts of tungsten do evaporate and condense on the cooler glass surface. As more and more tungsten evaporates, the filament gets thinner and the glass gets darker. So darkening of the glass is a signal that the filament will soon break and the bulb will blow. We won’t have to worry about this much longer as tungsten bulbs are being phased out in favour of LEDs.
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