Apparatus 2, further comprising a membrane inserted in a wall of {HEAD ~ said reference cell}, said adjusting means comprising means for exerting a force on said membrane, and means for adjusting {TAIL ~ said force} 
Apparatus 2 , further comprising a membrane inserted in a wall of { HEAD ~ said reference cell } , said adjusting means comprising means for exerting a force on said membrane , and means for adjusting { TAIL ~ said force }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL