In Wikipedia article on “electric charge”, someone wrote that “We now
know that the Franklin/Watson model was fundamentally correct. There
is only one kind of electrical charge, and only one variable is
required to keep track of the amount of charge.”
These statements can cause confusions because Franklin coined the term
"positive" for vitreous electricity and "negative" for resinous
electricity. (He believed that "negative electricity" was simply a
lack of electric fluid.) Franklin was also not aware that positive or
negative ions can be transferred during rubbing or electrolysis.
Hence, the concept of one kind of electric charge may cause confusions
to the students.
On the other hand, some physicists or physics teachers believe that it
is impossible to define charge. But may not we define charge as a
measure of how the photon propagates? (A. Zee, QFT in a nutshell, p188)