Chemical and thermal methods have been used to estimate the free atom concentration in a nonequilibrium supersonic jet of activated nitrogen emerging from a direct-current, high pressure, glow discharge. Emphasis is given to differential catalytic probe techniques, several variations of which are discussed. For nonequilibrium dissociated streams of low stagnation enthalpy, a differential catalytic thermometer is described which, in principle, offers the advantages of: (a) Linearity of probe response; (b) sensitivity independent of catalyst activity, details of gas dynamic environment, and catalyst geometry. A simple thermometric probe has been constructed and its output has been studied over a small range of electrical power dissipation. Evidence is given in support of the view that the probe responds principally to the same component of activated nitrogen that is depleted by chemical reaction with ethylene and propylene. Both catalytic probe measurements and the limiting amount of hydrogen cyanide production yielded about the same atom concentration (mole fractions of about 2 per cent). For electrical discharges operating at higher power densities, radiation losses from the catalytic thermometer surfaces necessitate design compromises which reduce the potential accuracy of the thermometric technique applied herein. This difficulty can be circumvented by using cooled catalytic probes (calorimeters), one type of which is currently under investigation. In general, it appears that the aerodynamic uncertainties involved in the catalytic probe theories presented are minor compared to the physicochemical questions raised by the use of such probes in nonequilibrium environments. Particular attention must be devoted to the role of excited species (other than ground state atoms) on the accuracy of both differential thermal detectors and chemical reactivity methods of analysis.

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