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Volume 8, Issue 4, 01 April 1937
8(1937); http://dx.doi.org/10.1063/1.1710289View Description Hide Description
This article, the last of the series, deals with the properties of the solid surface, and with the influence of impurities and flaws on certain volume characteristics. There has been very little theoretical work in these fields on the basis of the modern views. Consequently, a considerable part of what we shall say will be more or less speculative. We present these ideas in the hope that they may be suggestive, and caution the reader against embracing them uncritically.
8(1937); http://dx.doi.org/10.1063/1.1710293View Description Hide Description
8(1937); http://dx.doi.org/10.1063/1.1710294View Description Hide Description
An a.c. arc, in order to restrike after passing through current zero, requires a potential considerably higher than the normal burning potential. This re‐ignition potential is investigated for short gaps in nitrogen, using pure graphite electrodes with spacings up to two mm and pressures up to 500 cm Hg. The re‐ignition potential is found to have two characteristics. One of these characteristics is followed for the arcs in which the cathode spot is maintained by field emission, and the other is followed for the ``thermionic'' arc. Relations are obtained between this re‐ignition potential and both the gas pressure and the gap spacing, with the arc current as parameter. These relations may then be combined into a single relation between the reignition potential and the product of the pressure and spacing. This gives a function that is of the same form as the Paschen law for the initial sparking potential of a gap. The constants of this function are dependent on the arc current, the gas in the space, and the constants of the circuit. That the constants of the re‐ignition function are not the same as those for the sparking potential law is to be expected from the very dissimilar conditions applying in the space. Extreme purity of electrode materials and of gas, as well as extreme care in obtaining the experimental results, are essential.
Rheological Properties of Asphalts IV Observations Concerning the Anomalous Flow Characteristics of Air‐Blown Asphalts8(1937); http://dx.doi.org/10.1063/1.1710295View Description Hide Description
A great many of the asphaltic bitumens used commercially possess anomalous flow properties to some degree. Although all asphalts are probably elastic in some measure, detection and measurement of this property are difficult in the cases of certain bitumens, especially soft ones where the permanent deformation obscures the elastic effects. Data obtained by means of the rotating cylinder type of viscometer at a low shearing stress show the presence in an asphalt of elastic fore‐ and after‐effects. Frequently, when an asphalt is subjected to increasing shearing forces, a ``threshold'' value is reached above which the ``viscosity'' at a particular stress decreases with time until a constant value is obtained. If an asphalt is subjected to a high shearing stress (above the ``threshold'' value) in a rotating cylinder viscometer and the stress is reduced suddenly to a low value, the viscosity will increase gradually even though the asphalt is being worked constantly. Another manifestation of thixotropy is the age‐hardening of asphalts. This increase of viscosity with time, which varies greatly with different asphalts, can be overcome by heating or mechanical working. Upon standing, the viscosity again increases with time. Also, it has been found that for many asphalts the ``viscosity'' diminishes with increased shearing stress. The extent of these anomalous flow characteristics is dependent upon the source of material, the method and extent of processing to which it is subjected, and the temperature and rate of shear at which the measurements are made. Using numerous kinds and consistencies of asphalt, correlations of the Bingham‐Stephens alternating stress, the falling coaxial cylinder, and the conicylindrical rotation viscometers have been made. Comparisons were made at essentially the same mean shearing stresses in all three instruments for a particular asphalt. Good agreement was found when the asphalts were essentially viscous. Deviations of the results obtained for the three instruments became greater as the anomalous flow properties became more pronounced.