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James Gleick writes in his Feynman biography (Genius) that

The difficulty concerned the electron, the fundamental speck of negative charge. AS a modern concept, the electron was still young, although many high-school students now performed (as Feynman had in Far Rockaway) a tabletop experiment showing that electric charge came in discrete units.

What's such a tabletop experiment that a high school student could conduct in the 1930’s?

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    $\begingroup$ Millikan's experiment I guess... $\endgroup$ – Ruslan Dec 26 '19 at 18:22
  • $\begingroup$ There are kits available to do Millikan style experiments that use solid micro-beads rather than oil drops which are slightly easier (and less messy) than the classic experiment. But even with those it is hard to collect a useful dataset in a classroom setting. And the instructions for them often (usually?) direct students to use the easier but less accurate "get it to hover" scheme instead of Millikan's "measure the terminal velocity both up and down" scheme. I suppose these will do to establish discreteness even if they provide only a very coarse measurement of $e$. $\endgroup$ – dmckee --- ex-moderator kitten Dec 26 '19 at 18:28
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    $\begingroup$ A CRT does pretty well at showing electrons have a discrete charge. Or various other simple apparatuses that accelerate and bend electron beams. $\endgroup$ – Jon Custer Dec 26 '19 at 21:17
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Millikan Experiment

Also know as the oil drop experiment, it consists in determine the ratio between the mass of the electron and its charge, but putting a oil drop “fluctuate” in the presence of an electric field.

By finding the equilibrium position of the oil drop (consider the gravity force, the electric force and the hydrostatic force that can mainly affect the equilibrium) and by Newton’s Second Law it is possible to came up with such ratio.

Find more here: https://doi.org/10.1119/1.5126819

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  • $\begingroup$ Hardly a "tabletop experiment" experiment, IMO. $\endgroup$ – Gert Dec 26 '19 at 18:49
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    $\begingroup$ @Gert: Huh? Typical commercially available kits for that experiment fit quite easily on a typical teaching lab bench-top. It's not like high-voltage supplies are a problem these days. $\endgroup$ – dmckee --- ex-moderator kitten Dec 26 '19 at 18:59

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