The chosen reference frame determines whether an electromagnetic phenomenon is viewed as an electric or magnetic effect or a combination of the two. Authors usually derive magnetism from electrostatics when special relativity and charge invariance are taken into account. The Feynman Lectures on Physics (vol. 2, ch. 13–6) uses this method to derive the magnetic force on charge in parallel motion next to a current-carrying wire. See also Haskell and Landau.