the surface current u03b4i of tube 14 must be kept away from the magnetometer this includes the field from the individual wires 12 and 13 and from sources external to the device therefore a surrounding magnetic shield 18 is shown in fig1 in surrounding relation to the superconducting tube 14 to provide shielding from such fields referring to fig2 a and 2b there is shown a 77 u00b0 k ccc device constructed in accordance with a preferred embodiment of the invention the device which is generally denoted 20 comprises a set of two pairs of tube like superconducting shields 22a 22b and 24a 24b wherein each tube provides an unbroken unpenetrated layer of superconducting material which supports a persistent current at 77 u00b0 k suitable materials for the shields include bulk ceramic hts materials and thick film ceramic hts coatings applied to a substrate material experiments have shown that the length of these shield tubes should be at least 20 times the interior diameter or largest interior length dimension in cross section of the tubes 22a 22b and 24a 24b further the shield wall thickness should be minimized to reduce the overall size of the device 20 windings 40 are wound between the pairs of tubes such that referring to tubes 22a and 22b as typical the windings are formed in 34 turns 34 or loops having portions which extend along the tubes and expose portions indicated at 42 which extend between the tubes since it is necessary that a minimum length of each winding 40 should be unshielded to minimize unwanted magnetic fields the lowest possible number of shields made of straight sections is two each ccc winding 40 is made up of.