Letter from [Alan Archibald Campbell] Swinton, to Sir Henry Jackson Returning Jackson's notes of experiments with a shielded D. F. coil, commenting on his interest in the topic [enclosures not present]. Does not understand Jackson's explanation of the heterodyne effect as he does not see how incoming waves of any given frequency can excite waves of another frequency, even in a circuit which is out of tune. 31 March 1921