Today I finally corrected a gross error in the image of the golden leaf electroscope on the wikipedia page. Curiously the image stayed almost 20 years unaltered on wikipedia. This is high school experimental setup.
A golden leaf electroscope can only show repulsion between negative charges, never between positive charges.
My comment on the incorrect image:
https://commons.wikimedia.org/wiki/File_talk:Gold_Leaf_Electroscope.png#c-Arjen_Dijksman-20260923083600-Sign_of_charges_on_the_rod
I followed up this question on Physics Stack Exchange.
A few insightful quotes:
In A Treatise on Electricity and Magnetism, Vol. I, §63, Maxwell writes:
“We may therefore regard a body not electrified as virtually charged with equal and opposite charges of indefinite magnitude…”
and then:
“It is manifest, however, that this way of regarding an electrified body is entirely artificial…”
Maxwell then makes the important distinction:
“When we speak therefore of a body being charged with a quantity \(e\) of electricity we mean simply that the body is electrified…”
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Mach first constructs the familiar two-fluid picture:
“Let us picture to ourselves, now, two different electrical fluids, a positive and a negative fluid…”
and describes like fluids as repelling and unlike fluids as attracting.
Then comes the wonderful sentence:
“We must not seek in nature for the two hypothetical fluids which we have added as simple mental adjuncts, if we would not go astray.”
And he follows it with:
“Coulomb's view may be replaced by a totally different one, for example, by that of Faraday…”
and finally:
“…the most proper course is always, after the general survey is obtained, to go back to the actual facts, to the electrical forces.”
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There is an 1881 Helmholtz discussion that is also striking. He says:
“We shall try to imitate Faraday as well as we can by keeping carefully within the domain of phenomena…”
He then says that when he calls positive and negative electricity “substances of opposite sign,” he means only the empirical fact that one does not appear or disappear without the other appearing or disappearing nearby.
And then:
“I see very well that this assumption of two imponderable fluids of opposite qualities is a rather complicated and artificial machinery…”
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When we say this has been experimentally established for centuries, are we sure we're distinguishing the experimental observation from the theoretical symmetry? Maxwell himself called the picture of an uncharged body as containing equal positive and negative charges ‘entirely artificial,’ and Mach warned that we should ‘go back to the actual facts, to the electrical forces.’
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‘It has been proven for centuries’ — or has the symmetry simply been built into the description? Maxwell called the underlying picture ‘entirely artificial,’ and Mach warned us to go back to ‘the actual facts, to the electrical forces.’
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