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