The contraction of spring 20 causes needle 14 to be retracted into {HEAD ~ the void space} 15 in {TAIL ~ sensor base unit} 18 
in
The contraction of spring 20 causes needle 14 to be retracted into { HEAD ~ the void space } 15 in { TAIL ~ sensor base unit } 18
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH