Finally found Asimov's book hiding in the loft, and it was Ben Franklin's original guess notation for + & - that was chosen to set polarities. Another point, Scott, if the strokes are pulsing rapidly- do they have inductive effects on power lines close by, (apart from actually hitting them?). I assume that there are also magnetic fluxes occuring in a stroke, but can't find any references to induction effects on the net.
Alan
