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