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Edison effect for Incandescent Bulb?

  1. Jan 12, 2014 #1

    I would like to ask a question on incandescent light.


    From the Thermionic emission (Edison effect), heated tungsten filament emits electrons that could be collected by an anode (like a foil connected to positive voltage).

    The wiki also mentions that in order to facilitate thermionic emission, tungsten is often treated with mixture of barium, strontium and calcium.

    Does this emission happen in a normal incandescent lamp? In the bulb which Edison discovered the effect, there is a plate(foil) inserted into the bulb from the base, this is absent in normal bulbs. Does the tungsten filament still emit electron in this case? If it does, where would the emitted electron go? Without an anode collector, will they be suspended in the vacuum (assume a vacuum bulb) space inside the bulb?

    This further brings up a question to the energy balance (conservation) question in a vacuum bulb:


    This is the fundamental equation for all tungsten filament temperature calculation when the inside of the bulb is vacuum, and appears in Irving Langmuir’s 1936 paper and numerous others.
    Do we need to consider the thermionic effects for an ordinary incandescent bulb? How should we modify the equation above?


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    Last edited: Jan 12, 2014
  2. jcsd
  3. Jan 12, 2014 #2
    If the filament is hot enough, emission has to happen. However, as the emitted electrons have nowhere to go except back into the filament, and the filament becomes positive due to the emission, they will tend to go back to the filament. There is going to be some equilibrium amount of electron gas in the bulb but most likely not very significant to bother about.
  4. Jan 12, 2014 #3
    Definite figure?

    Dear voko,

    I agree very much with your answers and believe that an equilibrium will eventually (and very soon) be reached wtih very small amout of electron gas actually suspended in the vacuum space within the bulb.

    There is another Physical Review paper (Vannevar Bush, MIT, 1927) which also dealt with thermionic emisson on tungsten filament.

    Do you know where can I get some definite quantitative figure/data on the amount of electron emitted and henceafter remain in the vacuum bulb space?

  5. Jan 12, 2014 #4
    I would actually be surprised if somebody researched electron emission in an incandescent lamp. On the other hand, the theory required for that is part of the theory required to analyze and design vacuum tubes, and a lot has been written on that. Hopefully that should give some ideas on what to look for.
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