Bullard, Sir Edward (1907-1980), an English geophysicist, made a major contribution to the study of Earth's magnetism and the generation of heat inside the planet. He theorized that Earth's magnetic field results from convection currents in the molten (melted) part of Earth's core. Convection currents are patterns of circulation in a fluid that result from the tendency of warm material to rise and cool material to sink. Bullard suggested that convection currents in the core produce electric currents that generate the planet's magnetic field. See Magnetism.