as a yoke is positioned relative to the modules so that magnetic flux generated by the modules is coupled into the flux guide application_number US 47640590 A description the principle behind the invention is illustrated in fig1 in which an array of nine modular solenoids made from high temperature superconducting material is shown this is equivalent to the circuit shown in fig1 b it should be noted that each module must possess the same number of ampere turns as the required equivalent circuit in the case of the c magnet described with reference to fig3 the number of ampere turns needed to drive flux across the air gap of the c magnet is equ1 where ni ampere turns the construction of a module 1 is shown in more detail in fig2 each module 1 is formed from a number of spirally wound sections of a high temperature superconducting material in the form of a tape the sections 5 being stacked together concentrically a suitable material is yba 2 cu 3 o 7 made by du pont electrical contacts are made with the sections 5 via an inner copper cylinder 6 which is mounted in the bore 7 of the module and an outer copper cylinder 8 which contacts and surrounds the sections 5 in practice each module is immersed in a liquid nitrogen bath so as to reduce the temperature of the material sufficiently to achieve superconduction for clarity the cryostat arrangement has been omitted from the drawings the arrangement shown in fig1 can be adapted to produce homogeneous magnetic fields in working volumes positioned at a variety of locations commonly the working volume is at the geometric.