In 1956, Paul Zoll and colleagues published in the New England Journal of Medicine the first successful demonstration of defibrillation in human patients via electrodes applied to the skin of the chest, with no surgery required. It surpassed the limit of Claude Beck's earlier work (1947), which required a surgically opened chest. Zoll's device used high-voltage alternating current (AC) and was refrigerator-sized, necessarily connected to hospital mains power — a limitation Lown and Berkovits would resolve in 1962 with direct current. Zoll's work demonstrated defibrillation was viable non-invasively, opening the era of external defibrillators.