The current is automatically reversed at the point of {HEAD ~ zero motion} of the reciprocating mechanism during {TAIL ~ each rebound} 
The current is automatically reversed at the point of { HEAD ~ zero motion } of the reciprocating mechanism during { TAIL ~ each rebound }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL