wherein {HEAD ~ an exciting current} is flown through {TAIL ~ the exciting coil} so as to reverse its polarity at a periodical interval 
is flown through
wherein { HEAD ~ an exciting current } is flown through { TAIL ~ the exciting coil } so as to reverse its polarity at a periodical interval
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH