The distal ends of the electrodes are positioned at {HEAD ~ a central volume} of {TAIL ~ the airtight vessel} 
of
The distal ends of the electrodes are positioned at { HEAD ~ a central volume } of { TAIL ~ the airtight vessel }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL