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