• The uncer­ tainty in the new value is 1.7 ppm (stan­ dard error), a factor of 5 smaller than the uncertainty in the best previous value. The gas constant was determined from measurements of the speed of sound in argon as a function of pressure at the temperature of the triple point of water. The speed of sound was mea­ sured with a spherical resonator whose volume was determined by weighing the mercury required to fill it at the temperature of the tri~le point. The molar mass of the argon was determined by comparing the speed of sound in it to the speed of sound in a standard sample of argon of accurately known chemical and isotoptic composition.

Measurement of the Universal Gas Constant R Using a Spherical Acoustic Resonator

The Mariner 6 and 7 UV spectrometer experiments observed intense emissions from CO, O, and CO2(+) in the Martian airglow. Analysis shows that they are excited predominantly by the absorption of solar EUV photons by CO2 and constitute a major energy-loss mechanism for the thermosphere. Models of the thermospheric temperature profile and the airglow layer that demonstrate the effects of neutral chemistry and ionospheric composition are developed. With their aid, the observed CO Cameron-band emission scale height of 19 (plus or minus 4.5) km is shown to suggest an exospheric temperature of 315 (plus or minus 75) K. Consideration of other data suggests a 'best' value of about 350 K. Within the uncertainties in the excitation efficiencies and in the thermospheric cooling mechanisms, the observations are consistent with the measured electron density. There is no indication in the data that the ionosphere is modified by the solar wind below 200 km.

Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Implications of CO2 +, CO and O Airglow

A spherical acoustic resonator has been used to redetermine the universal gas constant R with an uncertainty of 1.8 ppm (standard deviation). To accomplish this, three subtasks were completed. (1) The volume of a spherical shell was determined by weighing the mercury required to exactly fill it at the temperature of the triple point of water, 273.16 K. (2) With the resonator filled with commercially supplied argon, the resonance frequencies of the radial modes were measured as a function of pressure. Using our theoretical model for the cavity, the frequency measurements were combined with the mean resonator radius determined in subtask (1) to obtain the speed of sound in commercially supplied argon. (3) Finally, the speed of sound in the commerically supplied argon was compared to the speed of sound in a “standard” sample whose chemical and isotopic composition was accurately established.

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Measurement of the universal gas constant R using a spherical acoustic resonator

The shock tube as wave reactor for kinetic studies and material systems

The physical optics of the laser-schlieren technique for the measurement of rate processes in shock waves is examined in detail. The method utilizes the Kirchhoff-Huygens integral with the usual thin lens, paraxial, and Fresnel approximations, all of which are appropriate for the typical laser schlieren experiment. The resolution and sensitivity of the technique are defined for all detector separations, and a reliable method for locating the time origin in the schlieren signal is provided. Diffraction is found to have a significant effect on the shock front generated signal, and geometrical optics treatments of this signal are shown to be inadequate.

Physical optics of the laser-schlieren shock tube technique.

Vibrational relaxation has been studied in CO2 and CO2–Ar mixtures over the temperature range 360°–3000°K using a laser–schlieren method. The purpose of this investigation was to compare the results obtained by this technique with those deduced using a Mach–Zehnder interferometer and to provide further evidence as to whether relaxation times depend upon how far the system is from equilibrium as well as on the translational temperature. Relaxation times measured by these two methods agree well. The laser–schlieren method is the more satisfactory for measuring the rate of relaxation and the Mach–Zehnder for finding the total density change during the process. These new results support the view that relaxation times do not depend upon how far the system is from equilibrium. The measurements using CO2 and CO2–Ar mixtures show that CO2 is about five times as effective as Ar in causing relaxation at 360°K and two times as effective at 2000°K.

C. J. S. M. Simpson

Papers

Erratum: Vibrational Relaxation in CO2 and CO2–Ar Mixtures Studied Using a Shock Tube and a Laser–Schlieren Technique