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