{HEAD ~ The first unit} is connectable to a first voltage source and the second unit is connectable to {TAIL ~ a second voltage source} 
{ HEAD ~ The first unit } is connectable to a first voltage source and the second unit is connectable to { TAIL ~ a second voltage source }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL