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Discovery of electrons - How did Thomson know they were particles?

  1. Dec 15, 2012 #1

    I have been having some fun in the last couple of days by looking at the details of historically significant experiments. One such is Thomson's discovery of the electron, in which he concluded that cathode rays were, in fact, composed of negatively charged, discrete particles. But as far as I can tell, his experiment (refresher - he put a cathode inside a glass tube under vacuum, the glass emits light where it is struck by the resulting cathode ray. Thomson used a magnetic field to divert the ray, indicating that the ray itself was charged) does not show that the ray is composed of discrete, negatively charged particles - it could just as easily be a continuous beam of negative charge.

    How did he conclude that the beam was discrete, rather than continuous?

    I want to make clear that I am not questioning the existence of electrons as particles. I know of plenty of evidence for that. This is primarily a historical question - how did Thomson, who did his experiment in the 1800s, figure out that a cathode ray was composed of particles?

    Thanks for any help you can give.

    -HJ Farnsworth
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  3. Dec 15, 2012 #2
    Thompson had already measured the charge on an electron by a forerunner of Millikans experiment.
    Thompson's 1897 experiment passing a cathode ray through crossed magnetic and electric fields established the velocity

    v= E/B

    This was found to be considerably less than the known value of c so Thompson concluded that the cathode rays were not waves.

    His experiment also measured e/m and thus m for the 'particle'. This was revolutionary because at that time it was assumed that the hydrogen atom was the lightest of all particles.
  4. Dec 18, 2012 #3

    Philip Wood

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    An interesting question. I suppose a continuous stream of electrically charged matter could be split by imaginary boundaries into imaginary portions with a given ratio of charge to mass, postulated to be the same for all portions. Then each portion could be treated as a particle, and the Lorentz force formulae which Thompson applied with consistent results would hold good. However, why should a continuous unquantised stream of electrically-charged matter have a consistent charge/mass ratio throughout? A more natural explanation would be that the stream is physically quantised into individual identical particles. [There were precedents: the idea of matter being quantised into atoms had been around for a long time, and had been accepted by British scientists far more than by German scientists. I believe, though, that Philipp Lenard shared Thompson's particle approach.]
    Last edited: Dec 18, 2012
  5. Dec 18, 2012 #4


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    I suppose that the atomic nature of matter had already been established by chemists so it wasn't such a leap to think of electrical charge as being quantised too.
  6. Dec 18, 2012 #5
    Hi everyone, thanks for the responses.

    I agree with Studiot's point that Thomson's manipulations with crossed electric and magnetic fields giving v<c established that the ray is not a light wave. After that post, I also read Thomson's forerunner to Milikan's experiment, where he finds e/m based on the at-the-time-known fact that water droplets form around ions. Had I been around then, I think I would have taken these as hints, rather than indisputable evidence, that the ray was composed of particles. Alternatively, it could have been composed of a continuous medium with a given q/m ratio - essentially, a line (or area or volume) with charge density λ and mass density ρ.

    Philip Wood, I read your comment a few minutes ago, so I just started thinking about it.

    I guess my first instinct is to ask, "why not?". I'm not sure that 1800s me would have regarded one explanation as more natural than the other. I read Thomson's paper, though, and he certainly did regard it as more natural, like you and Studiot. The precedent of quantized matter is a really good point, because it helps to explain why Thomson's mind might have gone in that direction in proposing an explanation for his experimental results.

    Here's what I'm thinking right now - this is one of those situations where history gives the credit for a discovery to a single event, when in reality that event represents more of a key step, but not the entirety, of that discovery (sort of an Edison light bulb situation). Thomson did an experiment, and proposed a model for it. The model was simple and explained the results of his experiment, as well as results of previous experiments, as well as results for experiments that came afterward. This last point is the most important, because it is what would have led to his discovery being universally accepted in time.

    We credit Thomson with discovering the electron because he did an experiment, and proposed the electron as the explanation for his experiment. But it is inappropriate to say that he established beyond a doubt the existence of such particles - that certainty probably came more slowly, as more experiments were performed which could be explained by Thomson's model.

    Does this sound like a reasonable answer - or am I an utter fool!?

    (Incidentally, the situation with Einstein and the photon seems similar to me. His quantized-light theory seemed to fly in the face of a long-settled debate regarding the nature of light. I don't think it was fully accepted until Compton's scattering experiments later. Historically, though, the discover of light-as-a-particle is most significantly given to a combination of Planck and Einstein).
  7. Dec 18, 2012 #6


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    You would have to read a lot of the papers published at the time in order to see how the modern idea developed. I can imagine that there was a lot of heated argument at the time. (Things never change!)

    It's true that these things are always attributed to just one Scientist -even Nobel prizes have a limit of three recipients per award, I believe. That's why there is such secrecy and a rush to publish first. Egos rule.
  8. Dec 18, 2012 #7

    Philip Wood

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    You are certainly not a fool, but are clearly thinking deeply about this issue. [Do you know the story of Russell telling Wittgenstein that he should not become an aeronaut?]

    I'm the first to admit that my attempt to make the continuous stream hypothesis implausible was hand-waving at best. I suspect that the British predilection for particles (Dalton's atoms, Maxwell's molecules and vortices...) may be more relevant.
  9. Dec 18, 2012 #8
    As I understand the experiments of Townsend (1897) and Thompson (1898) they were able to establish the quantisation of charge by their experiments.

    The experiments involved condensing water vapour around individual ions and they found that every ion had the same charge - there was no range of values.

    It was this that led Thompson to the idea of a unit particle of charge viz the electron. He subsequntly proposed the currently apt Christmas pudding (double pun intended) model of the atom as having a positive sponge(with distributed positive charge) with small negative electrons embedded in it.

    This was soon supplanted by the Rutherford & Bohr atoms.
  10. Dec 18, 2012 #9
    Hi sophiecentaur, sorry I missed your comment earlier. You're right about having to read a lot a papers, I think I've gone about as far down this rabbithole as I'm going to.

    Thank you. I vaguely recall that Wittgenstein asked Russell whether he thought he was good at philosophy, and Russell essentially told him yes by saying he shouldn't be an aeronaut. But when I type it like that, the story seems dull, and I remember thinking it was a really interesting interaction between a teacher and his student when I first heard it, so I think that I'm missing something. I don't think I will pursue this historical issue with an intensity to match Wittgenstein's philosophy :)

    If that's the case, then it would certainly have been enough to convince me. My understanding, which could be wrong/from a source that got it wrong, was that Thomson and Townsend's experiments involved averaging, rather than individual droplets. But I think I've gotten this issue to where I want it: Either measurements of individual droplets all yielded the same result, in which case a self-contained set of experiments showed to a certainty that the cathode rays were composed of particles, or Thomson's experiments and model heavily hinted toward this, and subsequent experimentation verified it.

    Thank you all for the help in digging into this neat piece of science history, it was quite fun for me.

    -HJ Farnsworth
  11. Dec 20, 2012 #10
    there are some simple physical tests in the history of physics to determine whether we are dealing with particles or waves (or something else!!!)
    If it is a wave property then diffraction and interference should be detected.
    If it is charged it is a particle
    If neither of these provide an answer then further investigation is needed.
    Further investigation leads to the conclusion that particles can show wave properties and waves can show particle properties.
    You pay your money and make your choice.
    Physics is great
  12. Dec 20, 2012 #11

    Philip Wood

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    I believe the question is not about particles vs. waves, but about particles vs. continuous fluid. Why should Thompson have assumed particles rather than continuous fluid?
  13. Dec 20, 2012 #12
    Weren't 'fashes on a screen' also detected in similar experiments at the time?
    would this not imply particles?
    Didn't someone else also suggest a plum pudding about the same time?
  14. Dec 20, 2012 #13

    Philip Wood

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    I thought that in Thompson's experiments individual flashes were not seen (only glows), but I may be wrong.
  15. Dec 20, 2012 #14
    At the start of the 20th century the great discoveries in physics were explained in terms of 'particle' physics. Thomson's ideas about atomic (particle) structure produced His 'plum pudding' model of an atom, it seems natural to believe that any 'beam' would consist of a stream of particles, especially if it could be deflected by magnetic and electric fields.
    Rutherford's discoveries in radioactivity (a few years later) also lead to the logical conclusion that particles were the best/logical explanation.
    Today, of course, we have a wider view.
  16. Dec 20, 2012 #15

    Philip Wood

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    I think the precise sequence of discoveries is important here. As you say, Rutherford's discoveries came a little later. The question is, why, back in 1897, did Thompson choose particles rather than a continuous fluid?

    It's time I shut up on this subject. I haven't got the mass of detailed historical knowledge needed to comment authoritatively.
  17. Dec 20, 2012 #16
    I would say that even in 1897 the discussion was between waves or particles. After all I imagine that back then a continuous fluid would be seen as a stream of particles....gases, liquids etc.
    Was light a wave? (Huygens said so) or a stream of particles (Newton said so). The answer was determined by measuring the speed of light in different mediums and in Young's interference experiments. (Huygens was 'correct').
    I think it is important to realise that no one person or idea was either correct or wrong.
    Individuals brought ideas together to produce the best explanation at the time.
  18. Dec 20, 2012 #17
    I still think you miss the point of the water condensing on ions.

    If there was a range of charge values you would have localised concentration of charge, just as with the argument about charge spreading out over the surface on a conductor which argument was certainly known at the time. the evening out of this charge could be expected to produce some observable effect, no such effect was observed or reported.
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