Experiment 4. The Triple Point A BC The vacuum manifold diagramed schematically in the ClassPak writeup is shown here in the same perspective, with stop- cocks labeled the same. A isolates the manifold from the pump, while B controls the vacuum to the sample cell. C normally stays shut throughout the run; it can be opened to admit air to the manifold, or to connect other devices, like pressure gauges. P gauge

2 In our current setups, the sample receptacle is fully enclosed within a larger glass cell, which, under vacuum, provides good insulation from the surroundings. The thermistor feeds up from below and is embedded in special epoxy to achieve good thermal contact with the glass and sample. The first step in the experi- ment is to carefully evacuate the system, after which the sample is “freeze-dried” under vacuum. It is essential that the sample not “bump” while it is being degassed and cooled. If it does, and water is splashed on the cell walls, the run is over!

3 In case we should need to use an earlier design of sample cell, the lower part of that cell is shown enlarged here. The thermistor probe is mounted through the knurled plastic nut; the copper wire surrounding the probe is there to facilitate thermal equilibration throughout the sample. Although this design has worked O.K. for years, it is unfortunately prone to leak air through the O-ring seal. If your team should work with one of these cells, we will provide the special instructions needed for its use.