The original paper by J.J. Thompson, where he exposes his experiment to measure $m/e$ can be found here. (Note that Thompson actually measured $m/e$ whereas the modern discussion is in terms of $e/m$.) It seems clear from the text that distances are measured in centimeters, and voltages in Volts, but as far as I can tell, Thompson does not provide units for magnetic or electric fields, or charges.

According to tables 1-3 on page 306 of the journal, Thompson found consistent values of $m/e$ in the range of $\sim 10^{-8}$, with no units given. The current accepted value of $m/e$ is $5.68\times 10^{-12}$ kg/C so clearly there's some conversion factor somewhere.

Thus: What units of electric field, magnetic field, and charge did Thompson use in his historical experiment?

  • $\begingroup$ My guess: at page 295 the capacity is measured in microfarad $\endgroup$
    – user6530
    Mar 23, 2022 at 11:34
  • $\begingroup$ Some info in chemeducator.org/bibs/0002006/00020149.htm - two things to note, first the author notes that Thompson used electrostatic units, and second, that the value given in the 1897 paper is entirely outside the range of later papers. $\endgroup$
    – Jon Custer
    Mar 23, 2022 at 13:24

1 Answer 1


J. J. Thomson was a brilliant physicists but when it comes to units, he just assumes that the rest of the mortals also know what he is talking about (perhaps a typical problem with overly intelligent people). In his landmark paper, he does not bother to mention units of his quantities at all. So one has to resort to modern commentaries from reliable authors.

You can start your journey from here: Discovery of the electron: commentary on J. J. Thomson's classic paper of 1897 DOI (https://doi.org/10.1080/09500830701305514)

The units in vogue in that time, were electromagnetic units (e.m.u.). There were also electrostatic units (e.s.u). This paper clarifies that the units for m/e in Thomson's paper were grams per electromagnetic units (e.m.u.) of charge.

e.m.u.: A system of electrical units formerly used in the c.g.s. system (see c.g.s. units). The e.m.u. of electric current is the abampere (all e.m.u. have the prefix ab- attached to the names of practical units). The abampere is the current that, flowing in an arc of a circle (1 centimetre in diameter), exerts a force of 1 dyne on unit magnetic pole at the centre of the circle. In e.m.u. the magnetic constant is of unit magnitude.



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