IV. LOW-TEMPERATURE PHYSICS

Production of Liquid Helium. The Collins helium cryostats have been given their most rigorous test to date. During the past quarter liquid helium was produced for the work of twelve undergraduate theses in addition to the requirements of the research program of the low temperature group. All of the requirements of the two programs were met on schedule. The following figures summarize the helium production. Number of runs 27 Running time 173 hours Liquid helium produced 364 liters Experiments serviced 116 Reduction of Cool-Down Time. The number of experiments serviced stimulated an effort to reduce the cool-down time. By using the auxiliary liquefying pot as a liquid air precooling line, in addition to the usual radiation shield line, the cool-down time was reduced to one hour. That is, one hour after starting the machine helium begins to liquefy. Fountain Effect Photography. Figures IV-1 and IV-2 illustrate the thermo-mechanical properties of liquid helium II. These photographs show the fountain more clearly than previously published fountain pictures. For the photography we are indebted to Prof. H. E. Edgerton, and to Mr. B. J. Diver of this Laboratory. R. P. Cavileer B. PRESSURE VARIATION OF SECOND SOUND VELOCITY IN HELIUM II Equipment capable of measuring the velocity of second sound in liquid helium II at temperatures down to 0.9 0 K and at pressures between the solidification line and the vapor pressure line has been completed. Measurements made thus far indicate that the second sound velocity rise observed at low temperatures either disappears, or occurs at temperatures below the range of the present equipment when the pressure is raised. Curves at 1 atmosphere and a