During {HEAD ~ reversal} the servo system which is driven into saturation senses the occurrence of zero motion of {TAIL ~ the drive mechanism} and reverses the direction of energization of the electromagnetic drive 
During { HEAD ~ reversal } the servo system which is driven into saturation senses the occurrence of zero motion of { TAIL ~ the drive mechanism } and reverses the direction of energization of the electromagnetic drive
OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH