The other of the further chambers contains 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 {HEAD ~ the first chamber} communicates with {TAIL ~ atmosphere} and an orifice through which the first chamber communicates with a source of gas at sub atmospheric pressure 
communicates with
The other of the further chambers contains 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 { HEAD ~ the first chamber } communicates with { TAIL ~ atmosphere } and an orifice through which the first chamber communicates with a source of gas at sub atmospheric pressure
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