Arbitrarily Positive about Labels

I have known the basics ever since I was in high school myself:  What we call positive and what we call negative is arbitrary; the truth is that electrons and protons must be oppositely charged but who wears which hat is just historical baggage; Ben Franklin is somehow the origin of this decision.

But today, while teaching about this in physics class, I speculated how the details of Franklin' glib macroscopic choice got translated into specifically assigning the microscopic negative to the electron and microscopic positive to the proton years later.  In the moment, I guessed it was Rutherford's fault for discovering the nucleus with alpha particle and thereby assigning it a positive charge.

Well, I was close but not quite right.  Here's a timeline based on an hour of internet sleuthing:

circa 1750 Ben Franklin proposes calling an excess of electric fluid "positive" and the lack of it "negative".  Somewhat arbitrarily, he determines which direction this hard-to-detect 'fluid' is moving with his experiments isolating charge in Leyden Jars.

Leyden Jars

1799 Alessandro Volta, looking to disprove's Galvani's proposed elettricità animale (from twitching frog legs), invents a primitive battery.  In confirming the flow of current with his electrometer, he conforms to Franklin's idea of positive charge flow. 

Voltaic Pile

Volta's Electrometer

1820 Hans Oersted noticed a deflection in a compass needle during a demonstration involving current during a lecture.

In this way, the direction of forces on conventional current (positive charge movement) in a magnetic field are determined. 

1879 William Crookes discovers that cathode rays from his eponymous tube are deflected the opposite way of conventional current, hence cathode rays must be negative.

Crookes Tube

1897 J. J. Thomson discovers that these cathode rays are made of electrons

 

In 1911 Ernest Rutherford discovers the nucleus and knows that it is positive by its deflection of alpha particles (which he discovered where positive by 1908 by their deflection in the same direction as conventional current!) But the deal was already sealed in 1897 so I was off by a couple of Nobel prizes (1906 Thomson; 1908 Rutherford)

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the TL;DR version:

Electrons are negative because they deflect opposite of current in a magnetic field and current is the movement of positive charges because Franklin said that what was moving in and out of his Leyden jars. 

The irony is, of course, that rather positive charges moving in and out of his jars, Franklin was really noticing negative charges moving out and in.  No big whoop, we had to start labelling those charges at some point and it was always going to be arbitrary.

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Bonus Fact:

Franklin used the terms Positive and Negative because their original meanings were "placed" and "denying" respectively.  Makes sense if you are thinking about an electrical fluid accumulating or lacking.  

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P.S.

If you think the plus and minus stuff is surprisingly arbitrary, wait to you find out what physicists did when they had to name the three-charge system for the strong nuclear force (1973):

Red Green and Blue Quarks and Gluons

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P.P.S.  1964 Gell-Mann named the constituents of neutrons and protons, in part, from a line from Finnegan's Wake:

"Three quarks for Muster Mark!
Sure he hasn’t got much of a bark
And sure any he has it’s all beside the mark."  

-James Joyce