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