Impedance defined?

Hi,



I was reading Charles Steinmetz "Theory and Calculation of Alternating Current Phenomenon" and on page 89 (can be found online easily) Steinmetz defines impedance as: Z = R - jX, but see it commonly defined as R + jX. I read on wikipedia there are two impedance equations for capacitive impedance and inductive (R - jX, R + jX), but Steinmetz doesn't mention either or suggest this is a special case of impedance, just that it applies for alternating waves.



So, did Steinmetz make a mistake in his definition? How do we know which impedance he is talking about?



I am confused because I doubt Steinmetz defined it incorrectly, yet don't know where the sign difference comes from? I get the wikipedia explaination of phase difference, but Steinmetz says its the impedance, I don't think he meant just capacitive or inductive.



EDIT: Looking further into the book, he only uses Z = R - jX throughout the whole book, seems to be the general definition, regardless of inductive or capacitive



Thoughts? Thanks.