"The proportionality factor σ (not to be confused with surface charge) is an empirical constant that varies from one material to another; it's called the conductivity of the medium. Actually, the handbooks usually list the reciprocal of σ, called the resistivity: ρ = 1/σ (not to be confused with charge density—I'm sorry, but we're running out of Greek letters, and this is the standard notation)."

This is the man who allegedly can teach physics to gerbils. I wish I could take a class with him simply for his wit and skillz at teaching.