Onnes, a Dutch physicist, discovered in 1911 that mercury loses all resistance to electrical flow when cooled to about 4 K; thus, a current once started will flow continuously. Such a phenomenon is known as superconductivity. At ordinary temperatures, metals have some resistance to the flow of electrons, due to the vibration of the atoms which scatter the electrons. As the temperature is lowered, the atoms vibrate less and the resistance declines smoothly, until the material’s critical temperature, Tc, is reached. At this point, the resistance drops abruptly to zero (figure 1). If an electrical current is started in a superconducting ring, it will continue forever.
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