An excellent question! I think a good answer will requiring making a correction to the page. Schiffer's book says that he (Schiffer) reformulated Barlow's law to state that the "current strength" varies inversely with the square of the length of the wire. I don't see how to reconcile this with the direct quotations of Barlow's law that are found in other places (like the Report on Mining, etc.). I'm guessing that the answer has something to do with Barlow's terminology, which we should explain in the article. Barlow's law could not have been about conductance (or resistance) in the modern sense of the word, because that concept wasn't established until Ohm's law.  —