said ferrous cylindrical wall having an outside surface, an inside surface, and a central axis, said outside surface being {HEAD ~ a convex}, curved, outside cylindrical surface, said outside surface being contiguous said inside ends of {TAIL ~ said pole pieces}, 
said ferrous cylindrical wall having an outside surface , an inside surface , and a central axis , said outside surface being { HEAD ~ a convex } , curved , outside cylindrical surface , said outside surface being contiguous said inside ends of { TAIL ~ said pole pieces } ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH