In high school and junior-high school in Japan, we read a' as "a dash". However, in universities, it is occasionally read as "a prime" due to American influence.
I am not native speaker of English; therefore, I explored dictionaries and found a description. The Oxford English Dictionary vol. XIII (1970) states that it "usually read as `a dash' " in the explanation of the word "prime". I feel something odd in this statement, but in the explanation of the word "dash" in OED vol. III (1969), I found that "a stroke or line (usually short and straight) made with a pen or like, or resembling one, so made ....". I am now realised the symbol ' should be recognized to be a short script.
I would like rather ask why American reads a" as double prime. Why the prime (= the most important) is not only one?