Superconductivity is characterized by a resistanceless flow of electrical current.
Subsequently, a number of metals like lead, tin and niobium were found to loose resistance as well at very low temperatures. The most widely used superconducting material was niobium which is superconductive below 9.2 K, and in application was usually cooled to 4.2 K by immersing it into liquid helium. He could show that despite the thin insulating layer, a current can flow from one superconductor to the other due to quantum-mechanical tunneling effects. Up to a certain value, this current flow is without losses. When the Josephson junction is in the resistive state it consumes energy which is drawn from the LC circuit dampening it.