grammatical-number
I have mostly seen "fluxes" but I could not find a definitive answer in some dictionary.
As it come from the French, I am enclined to think it is "flux" in the plural though.
Edit : I am talking about the flow of a quantity, as an air flux.
Example : "The sum of all air fluxes was zero".
Etymonline has this to say:
-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).
So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.
Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:
-ic
suffix forming adjectives
- of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
[...][from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]
[...]
-ics
suffix forming nouns (functioning as singular)
- indicating a science, art, or matters relating to a particular subject: aeronautics, politics
- indicating certain activities or practices: acrobatics
[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]
The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".
Well, this is an example of why Google NGrams isn't a precise indicator. When we compare the two directly, aircrafts simply can't get off the ground:
Now, this result is also flawed since it is impossible to separate uses of aircraft (singular) and aircraft (plural). It is also impossible to factor out typos (aircrafts vs. aircraft's) and so on.
The point is, don't read too much into what an NGram shows (or at least take the graphs with a grain of salt), because a great deal of the time what you wind up with is this sort of thing:
(BTW, the huge spike in the use of aircraft in the early '40s is almost certainly due to the air war in Europe and the Pacific.)
Best Answer
The entry for flux in the Oxford dictionaries does not show any use of
fluxin plural. It actually annotates some of the uses as mass nouns.However, the plural form
fluxesis often found in technical texts. Here's a quote from a very well-known text-book on Physics:If you still prefer to avoid the use of the plural form
fluxes, you could do so by replacingthe sum of fluxeswiththe total flux.EDIT
With @alex_reader's help, I've found that the oldest occurrence of
fluxeslinked by Google's n-gram viewer is: