for a shunt placed across the terminals of such a solenoid and the current source removed to achieve a persistent current in the solenoid it is common in the art to employ a persistence switch to control transitions between the persistent mode and the externally driven mode for a superconducting circuit a persistence switch is formally akin to a monostable relay the superconductivity of a shunt is destroyed returning the superconducting state of the shunt to the normal state thereby terminating the persistent mode a solenoid selector 14 is used to initiate or destroy persistent currents is a selected solenoid by controlling a selected one of the set of persistence switches schematically indicated by box 14 it will be understood that the persistence switches of the apparatus are distributed about magnet assembly 50 as appropriate for connection to the respective coils comprising assembly 50 fig2 shows a form of such a switch commonly employed for this purpose superconductor 20 connected across the input power terminals of a superconducting circuit not shown passes through a dielectric member 23 in which is also embedded a resistive heating element 24 when heating current is applied to the resistive element 24 the superconducting state of shunt 20 is destroyed when heating current is removed the shunt 24 is cooled by the coolant liquid helium.