Why Christmas Lights Can Get Hot: Electrical Safety During Holidays

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Fluorescent bulbs are gas discharge lamps. Photo by Alvimann.

Question:

“I have one vintage fluorescent bulb that I plugged in to see if it worked, and it did. After about 20 minutes I unscrewed it and the metal screw end was really hot. I tried it on another cord and same thing. The other bulbs did not get hot. Is that going to be a danger?”

To answer the question, we will describe the principle on which fluorescent lights are based, how the metal screw end is made, and talk about the possible causes for this overheating.

Fluorescent Lamps

Fluorescent lamps are gas-discharge lamps.

In these lamps, there is an internal glass case which contains an inert gas (i.e. argon or neon), mercury (Hg) in the vapour state, a cathode (a conductive solid wire/plate which can conduct electricity) and a powder of appropriate compounds (phosphors), deposited on the internal surface of the glass.

When a current passes through the cathode, some electrical charge is released; this will cause an increase in the energy of the mercury vapour atoms and, consequently, the emission of light.

The wavelength of the emitted light falls into the ultraviolet (UV) region and, therefore, it is not visible to the human eye. Phosphors convert the light to visible light; these are chemical compounds which can absorb UV light and then emit it at different wavelength, in the visible range (from about 400 to 800 nm).

Compact Fluorescent Bulbs

In the past, the most common fluorescent lights used had the shape of a tube; more recently, however, compact fluorescent lights (CFL) have become more common.

These are bulbs based on the same principle as fluorescent tubes, but with shapes more similar to traditional incandescent bulbs; in this way, CFLs can be used to replace the old incandescent ones.

In particular, screw-in CFLs have metal screw bases of the same dimensions as the incandescent bulbs; therefore, we can use them directly in all electrical household lighting appliances, without needing any additional parts or adapters.

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