Thomas Edison in 1883 noticed that electrical current flowing through a light bulb's filament could make the wire so hot that electrons boiled off, sailing through the vacuum inside the bulb to a metal plate that had a positive charge. Because Edison didn't see any way the phenomenon would help him perfect the light bulb, he only made a notation of the effect, which he named after himself. The effect sat on the shelf until 1904, when a former Edison employee, inventor John Fleming, went to work for the Marconi Radio Company. For his first assignment, finding a better way to receive distant radio signals, Fleming began experimenting with the Edison effect. He discovered that radio waves passing through an airless tube created a varying direct current, which could be used with headphones to reproduce the sound carried by the waves. Fleming named it the oscillation valve and applied for a patent. Marconi, though, chose another, less expensive technology: a crystal wave detector.

Here's what I understand. The electrons did the action of boiling off and then sailed through the vacuum inside the bulb and so on and so forth. So, the only obstacle to the perfect understanding of that sentence I have is deciphering what exactly boiling off means in that context.

  • I think the high temperature, which was produced by electric current, boiled off the electrons. As I understand it, the electrons from the wire/filament moved to the vacuum part of the bulb and then moved to the metal plate. Hence - the electrons boiled off. meaning, moved/removed. Maybe displaced is the closer meaning in this context?
    – shin
    Sep 2, 2015 at 9:48
  • to boil off thefreedictionary.com/boil+off
    – rogermue
    Sep 2, 2015 at 11:41

1 Answer 1


Answering this question is not just about English Language but also about physics and physics terminology.

Boiling involves heating something until it reaches a specific temperature where the liquid starts to change into the gaseous phase. It is often used in common everyday language to refer to the heating of water, but it can refer to a much wider physical process in involving heating.

When substances are heated the molecules become more agitated (or excited) and some fly off the surface. Many fall back. When many are so energetic that they leave and do not fall back we call this boiling. Boiling does not just cause molecules to become energised and fly off; this can happen to atom, ions and sub-atomic particles, such as electrons.

You may know that changing the pressure affects boiling. If you take water up a mountain it will boil at a lower temperature. The fewer gas molecules in the air permit the water molecules to leave the liquid and not fall back at lower energy levels, and thus at lower temperatures.

If you really lower the pressure, such as with a vacuum, which is almost zero pressure, many things will fly off the surface of heated materials. With metals, specifically, electrons will fly off when they are heated. This phenomena was what was detected by Edison.

This is an important process in various parts of physics and electronics. In particular the mechanism of X-ray machines and particle accelerators like LHC in Geneva.

Physicists will calculate such things as the mean-free-path of an electron in a vacuum at different temperatures. The study of boiling electrons is an important contribution to our modern world from the discovery you cited.

Therefore the phrase electrons boiled off is both an accurate description using the terminology of physics and uses a word we use in everyday language. In everyday language we might say boiling water to make steam. Another more mundane use of the phrase might be in cooking: we might say "boil off excess liquid when making soup".

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .