George Backus has made important, original contributions to mathematical geophysics over the last 30 years. In his doctoral research, under S. Chandrasekhar, he extended Cowling's theorem to show that an axisymmetric magnetic field could not be sustained in a sphere of conducting fluid with time‐dependent axisymmetric flow. In 1957, Backus and A. Herzenberg independently demonstrated that Elsasser had been correct in suggesting that dynamo action in a fluid sphere was possible if the axisymmetry were broken. This result forms part of the foundation of geomagnetic dynamo theory and still depends on the mathematical demonstration due to Herzenberg and Backus, as experimental tests remain inconclusiv