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