type it comprises a bipartite housing 20 including hollow coaxial generally cylindrical sections 21 and 22 of ferrite or other material of high magnetic permeability which contain windings 12 and 13 respectively housing 21 comprises an end wall or base 23 from which a peripheral wall 24 extends to a rim 25 and from which a central pedestal 26 extends in the same direction pedestal 26 is traversed by an axial hole 27 housing section 22 comprises an end wall or base 33 from which a peripheral wall 34 extends to a rim 35 and from which a central pedestal 36 extends in the same direction pedestal 36 is traversed by an axial hole 37 winding 12 is mounted in housing section 21 around its pedestal 26 and winding 13 is similarly mounted in housing section 22 about its pedestal 36 a suitable fastener 39 such as a nylon screw 40 passes through holes 27 and 37 to hold the sections in assembled relation with the rims and pedestals of the sections in apposition and contact as shown in fig3 a disc or washer 41 of ferrite or other suitable material is located in housing 20 between windings 12 and 13 to act as a magnetic shunt for this purpose pedestal 36 may be provided with an accurately machined shoulder 42 and rims 25 and 35 may.