Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

242 EXPERIMENT A L ME TH 0 D S FOR HETEROGENEOUS RE ACT I0 NSbe removed during bake-out of the tube and then accurately realigned if theinstrument is to function properly. Pressures below torr are required for thetube to operate correctly, so that the analyser extracts a negligible amount ofgas from a reaction system. Partial pressures down to lo-'' torr can be detected.For kinetic work a constant emission for the filament is essential263s264 . On ethat has been used successfully265 is shown schematically in Fig. 34. The balanceddifference amplifier (V3, 4, 5) has one output grid (V3B) referenced to a Zenerdiode and the other (V3A) sees a signal proportional to the electron collectorcurrent. The output of this amplifier feeds a saturable reactor, which controls thefilament voltage in order to correct for any change in collector current. R7 controlsthe loop gain, to make the control stable over the range 0-10 microamps emission.Capacitor c6 suppresses high frequency oscillations which otherwise occur at thehigh end of the control range. The circuit also provides other voltages necessaryfor operation of the omegatron tube and for bypassing of the emission regulatorduring outgassing.If oxygen is admitted to a pressure of 5 x torr, which is about the upper-=I- -Fig. 35. Diagram of the rf mass spectrometer tube (topatron) and its electrical control system(after Vttradi).