Electrical Resistances and Volume Changes up to 20,000 Kg./Cm ²

This note presents a brief summary of several phenomena at pressures up to 20,000 kg./cm.2 which will be described in full detail in Proceedings American Academy of Arts and Sciences.20,000 kg./cm.2 represents a considerable extension of range of my previous measurements, which usually reached to 12,000.I had formerly in one instance made accurate measurements up to 20,000, but the apparatus did not survive a single application of pressure, so that the expense of making similar measure- ments of other phenomena would have been prohibitive at that time.In the last few years, however, new grades of steel of much superior strength have been developed, so that the life of the apparatus subject to 20,000 is now long enough to justify the serious attempt to measure in the wider range such phenomena as experience in the 12,000 range have shown to be of probable special significance.As subjects for these first measurements over the extended range were chosen the electrical resistance of black phosphorus and tellurium, both of which show very large effects up to 12,000, and the pressure-volume-temperature relations of the alkali metals lithium, sodium and potassium, with especial emphasis on the thermal expansion.A number of modifications in technique were introduced, which are fully described in the detailed paper.In spite of all these modifications, measurements in the extended field remain much more difficult than in the original range.The essential difficulty is the short life of the apparatus, and the fact that an explosion may involve not only the loss of the pressure vessel itself but also of the apparatus within the vessel.In order to ob- tain the data of this note, five different pressure vessels had to be used; contrasted with this is the fact that it is now several years since a pressure vessel used only up to 12,000 has broken.Perhaps the most fundamental question of technique in the extension of range is the validity of the pressure measurements.The pressure gauge used up to 12,000 has been the manganin resistance gauge, in which pres- sure is measured by the change of resistance of a coil of manganin wire.AnI initial calibration of this gauge against an absolute gauge up to 12,000 showed the relation between change of resistance and pressure to the linear within 1/10% over this range.The limit of the absolute gauge is about 12,000, and pressures between 12,000 and 20,000 are measured by a linear extrapolation of the manganin gauge.