World War II fluxgate magnetometer : A device developed for military use during World War II went on to play a key role in one of the biggest breakthroughs in Earth science.
After the war, oceanographers refitted the portable fluxgate magnetometer, originally used to detect enemy submarines, so it could be towed behind research vessels to measure magnetic anomalies preserved in seafloor rocks. The measurements it enabled eventually led to the "magic profile," one of the most convincing pieces of evidence for seafloor spreading and, ultimately, the theory of plate tectonics, as per a report.
