Pressures in Discharge Tubes

Temperature and percentage dissociation of He, H₂, O₂, N₂, air, CO and CO₂ as functions of pressure and power input in discharge tubes. ---The increase in pressure of a gas in a discharge tube due to the discharge has been measured for pressures from about 0. 1 to 20 mm of mercury for helium, hydrogen, oxygen, nitrogen, air, carbon monoxide and carbon dioxide. The pressure increase is regarded as due to two chief causes, one, the increase in temperature of the gas, and another, the dissociation of the molecules of the gas into atoms or less complex molecules, and has been used to determine the temperature and the dissociation. Two discharge tubes were used in turn, one 300 cm in length and 9 mm in internal diameter, the other 80 cm x 34 mm. Pressures below 1 mm of mercury were measured by a striation gauge, which was a second discharge tube (joined to the main tube) excited by direct current calibrated so that the shift of the striations of the positive column with pressure was known. Pressures from 2 to 20 mm were measured with an oil manometer. With helium the temperature increment above the temperature with no discharge, about 300^, calculated from the pressure increased linearly with the power in the discharge tube, being about 11^ and 22^ with 500 and 1000 watts, respectively, in the long tube, and 14^ with 500 watts in the shorter tube. With moist hydrogen the pressure increments, corrected for temperature, gave for, the concentration of the hydrogen atoms (i. e. the number of atoms divided by the number of atoms plus molecules) values which increased rapidly with the power to about 400 watts for the long tube and pressures below 1 mm of mercury, being thereafter appreciably constant; the values of for 150 watts were 50 to 70% and for powers above 400 watts were close to 100%. For the shorter, larger diameter tube was not so great as in the long tube, due to the fact that the power per unit area was less. With dry hydrogen was less. All in all, the atomic concentrations were in exact accord with spectroscopic inferences. For oxygen rose to values as high as 60% at pressures below 0. 25. For nitrogen, carbon monoxide and carbon dioxide was close to zero. These conclusions are based on the assumption that few complex molecules were formed, such as H₃, N₃, O₃, etc. The results with condensed discharges were roughly the same as with uncondensed currents. The higher pressure measurements, from 2 to 20 mm, indicated discharge temperatures as high as 300^, but could not be used to determine becuse of uncertain temperature corrections. A new striation phenomenon with condensed discharges at these pressures is described.