The other of the further chambers contains {HEAD ~ a second electro magnetic control valve} fed both by said D.C. voltage and by a pulsed voltage, and operable to control both an orifice through which the first chamber communicates with atmosphere and an orifice through which the first chamber communicates with a source of gas at {TAIL ~ sub atmospheric pressure} 
The other of the further chambers contains { HEAD ~ a second electro magnetic control valve } fed both by said D.C. voltage and by a pulsed voltage , and operable to control both an orifice through which the first chamber communicates with atmosphere and an orifice through which the first chamber communicates with a source of gas at { TAIL ~ sub atmospheric pressure }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL