What is the plural of 'flux'?

Solution 1:

The entry for flux in the Oxford dictionaries does not show any use of flux in plural. It actually annotates some of the uses as mass nouns.

However, the plural form fluxes is often found in technical texts. Here's a quote from a very well-known text-book on Physics:

"If we now add Eqs. (3.14) and (3.15), we see that the sum of the fluxes through S1 and S2 is just the sum of two integrals which, taken together, give the flux through the original surface S=Sa+Sb." (The Feynman Lectures on Physics, Volume II)

If you still prefer to avoid the use of the plural form fluxes, you could do so by replacing the sum of fluxes with the total flux.

EDIT

With @alex_reader's help, I've found that the oldest occurrence of fluxes linked by Google's n-gram viewer is:

"There are some which become tarnished by fluxes, that contain the oxydes of lead." (The circle of the mechanical arts -- Thomas Martin, 1813)

Solution 2:

I worked for a company which supplied metallurgical chemicals internationally to steel making and foundry businesses. One category of products were known as 'fluxes', each with its own product name and specification.