ABSTRACT
The electrical resistance of a conductor is temperature dependent, decreasing with decreasing temperature as shown in Figure 9.1. If the temperature is reduced toward absolute zero in the cryogenic range (the range of liquid nitrogen to liquid helium), the dc resistance of certain conductors abruptly drops to precise zero at some critical temperature Tcr. Below this temperature, the superconducting coil requires theoretically zero voltage to produce enormously high current and the resulting high magnetic flux. In practice, a negligibly small voltage is needed to overcome the lead transitions from the coil to the room temperature voltage source. The coil itself needs no voltage to circulate a steady-state dc current in the coil, and the coil terminals can be shorted to continue circulation of the current forever. The circuit time constant L/R is now infinite, meaning that the current will continue to flow in the coil indefinitely. The coil is said to have attained the superconducting state with zero resistance. The energy stored in the coil then freezes and remains stored indefinitely.
