How The Mass Of an Electron Was Worked Out

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    Electron Mass
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Discussion Overview

The discussion centers around the historical methods used to determine the rest mass of an electron, exploring various experiments and their implications. It includes references to specific experiments such as the oil drop experiment and the charge-to-mass ratio measurements.

Discussion Character

  • Historical
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant inquires about the discovery of the electron's rest mass, suggesting that the forum may provide insights.
  • Another participant references the oil drop experiments conducted by Millikan as a key source of information.
  • A participant questions how the electron mass could be determined from the oil drop experiment, noting that the mass of an oil drop is significantly larger than that of an electron.
  • It is proposed that by demonstrating charge quantization, one could use an electron beam in a magnetic field to determine the charge-to-mass ratio (e/m), which could then be used to calculate the mass of the electron.
  • Historical contributions are mentioned, including J.J. Thompson's measurement of the charge-to-mass ratio in 1897 and Kaufmann's work on the velocity dependence of electron mass in 1901.
  • A summary is provided that attributes the measurement of the charge-to-mass ratio to J.J. Thompson and the measurement of the electron charge to Millikan, who also contributed to determining the electron mass.

Areas of Agreement / Disagreement

Participants present multiple viewpoints regarding the methods used to determine the electron mass, with no consensus reached on the specifics of how these measurements were interconnected or the implications of the findings.

Contextual Notes

There are unresolved questions regarding the relationship between the oil drop experiment and the determination of electron mass, as well as the historical timeline of discoveries related to the electron's properties.

Homewoodm01
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I was just wondering how the rest mass of an electron was first discovered, I've searched everywhere and think you guys are probably the ones that are most likely going to be able answer this question.
 
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The Millikan experiment was originally intended to show charge quantization. I wonder how one could determine the electron mass from this experiment at all because one oil drop has a mass that's by orders of magnitude larger than the electron's mass.

But if one has shown that charge is quantized one only has to shoot an electron beam of specific velocity into a magnetic field and then determine the radius of the circular orbit it takes (for filtering a certain velocity one uses crossed E and B-fields => I think this is called Wien filter) You get the charge to mass ratio e/m and from this you can easily calculate m.

Wikipedia seems to say that it was J.J. Thompson himself who measured the charge to mass ratio in 1897. According to one of my books a man named Kaufmann discovered the velocity dependence of electron mass in 1901 (four years before the formulation of special relativity).
 
Last edited:
In summary, J.J. Thompson performed the experiment which measured the charge-to-mass ratio of the electron. Millikan later performed the experiment which measured the electron charge, also determining the electron mass.
 
Thanks for your quick responses everyone
 

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