Index of content:
Volume 20, Issue 7, 01 July 1949
20(1949); http://dx.doi.org/10.1063/1.1698449View Description Hide Description
A gas‐water mixture, when expanded through a de Laval nozzle, acts as a compressible fluid. A general expression for the flow process in an idealized mixture is developed. The general thermo‐hydrodynamic relations for flow through a de Laval nozzle are developed. The magnitude of the velocity of an energy pulse or signal through the mixture is determined and is compared with the velocity of flow. Experimental results are compared with those predicted using the equations developed.
20(1949); http://dx.doi.org/10.1063/1.1698450View Description Hide Description
A new photographic technique is described, for observing velocity fields in water, induced by the entry of solid missiles. The results of a number of observations are given, and compared with theoretical formulas. The corrections involved in the method are analyzed theoretically, and the theory of the corrections confirmed experimentally. It appears that the method is well adapted to velocity fields 5–100 feet per second, created by objects several inches or more in diameter, but may lead to troublesome corrections at other speeds or with smaller objects.
20(1949); http://dx.doi.org/10.1063/1.1698451View Description Hide Description
In subtropical Africa dust is shown to be an active meteorological element, particularly below the T‐inversions common at 2000–3000 feet under Etesian conditions. Actual instability of the lower atmosphere is then not necessary for dust to rise, and mere mechanical stirring by day by winds of about 20 miles per hour or more is sufficient to produce severe sand storms over desert areas under such conditions.
A dust particle suspended in the atmosphere has been assumed to possess an ``air‐pocket'' or ``surrounding eddy'' of its own. The radius of the ``air‐pocket'' of a particle of radius ``a'' cm (less than 5 micron) is shown to be about 10a cm. It is doubtful whether one can assume the persistence of one and the same ``eddy,'' but with such an assumption made the average radius of the ``eddy'' is found to be 6a by observation, but on rejecting the assumption there appears to be a close agreement between theory and observation. The particles also share their excess of heat with their environment in a very small time, of the order of 10−2–10−3 second.
The amount of heat that can be directly supplied to the lower atmosphere by baked dust blown in a dust storm may, by itself, be sufficient to produce convection up to a height of 3000 feet or more. The same range of height of convection is also attainable in a cloud of dust by the absorption of solar radiation scattered or diffusely reflected within the cloud. In this respect it is suggested, though not fully verified, that the effect of suspended dust should be taken into account when discussing problems of upper‐layer instability in subtropical Africa.
It has also been observed that in the special cases of ``squalls'' the intensity of the storm, notably the poorness of visibility, is proportional to the time during which the squall reaches its maximum. The feature has been given a mathematical explanation.
The conclusion is that owing to increased absorption of solar radiation within dusty atmospheres, or to direct heating of the air by baked dust that may be blown by day in the warm season of the region, dust is found to play a part that cannot be ignored in modifying the processes of water vapor and heat transfer, by altering turbulent and radiative processes, and in modifying the properties of air masses invading the deserts.
20(1949); http://dx.doi.org/10.1063/1.1698486View Description Hide Description
It was found that the lower peak hardness and faster overaging of a Co‐containing Cu‐Be alloy quenched from a low solution treating temperature is associated with a large amount of grain boundaryprecipitation. A similarly large amount of grain boundaryprecipitation, coupled with low peak hardness value, is obtained if the alloy contains a small amount of Cr impurity, even if the quenching temperature is normal. The grain boundary reaction resulting from the Cr impurity can be eliminated by using high solution treating temperatures.
20(1949); http://dx.doi.org/10.1063/1.1698507View Description Hide Description
It has been widely assumed, without adequate experimental verification, that barium‐strontium oxide, as used in the oxide cathode, is an excess electronic semi‐conductor. Accordingly, the electrical conductivity of (Ba, Sr)O has been studied as a function of temperature before and after activation with methane, extensive precautions being taken to exclude spurious effects. The increase in conductivity obtained characterizes (Ba, Sr)O as a ``reduction'' semi‐conductor, and hence very probably as an electronic semi‐conductor whose conduction electrons arise from a stoichiometric excess of (Ba, Sr) atoms in solid solution.
A basic prediction of the semi‐conductor theory has been tested quantitatively with the finding that the electrical conductivity and the thermionic emission of a (Ba, Sr)O cathode are directly proportional through three orders of magnitude of activation; well‐defined chemical and electrical activation and deactivation procedures were used in obtaining this result. It may be concluded that activation represents an increase in the chemical potential of the electrons in the oxide, little or no change in the state of the surface occurring. It has also been found that deviations from the proportionality of conductivity and emission may be expected under conditions leading to inhomogeneity in the oxide, in agreement with the semi‐conductor theory also.
20(1949); http://dx.doi.org/10.1063/1.1698508View Description Hide Description
To obtain suitable conductivity for direct heating, tungstenpowder is added to sintered thoria cathodes. The thermionic properties of a cathode,sintered from a mixture of 67 percent thoria and 33 percent tungsten, were investigated. Emission constants were determined and the emission was found to be somewhat lower than that of cathodessintered from pure thoria.
Change of cathode activity with temperature was observed and studied in detail. The cathode showed high activity at temperatures which would bring about deactivation of ordinary thoriated tungsten and decreased its activity when the temperature was lowered to the range where thoriated tungsten would be activated. A theory is proposed for the mechanism of the change in activity, involving the production of free thorium due to reduction of thoria by tungsten,diffusion and evaporation of thorium from thoria. Evaporation of thorium as well as thoria was proved by the use of a tungsten filament probe which detected evaporation of materials from the cathode by the change of its own emission. The energy of evaporation of free thorium from thoria is estimated from the experiments to be 46600 cal./g‐atom. The energy of evaporation of thoria is found to be 184000 cal./g‐molecule. The behavior of the cathode is satisfactorily explained by the proposed theory.
20(1949); http://dx.doi.org/10.1063/1.1698509View Description Hide Description
A furnace has been designed which enables one to carry out electron diffraction studies of thin films at high temperatures. Electron diffraction patterns are obtainable at temperatures up to 1100°C. It is possible to bring the furnace up to 1500°C, but at temperatures over 1100°C the photographic plates are fogged by light emitted from the furnace. With the use of this furnace,recrystallizations and phase changes in thin metal and alloy films can be studied at the temperatures where these phenomena occur.
20(1949); http://dx.doi.org/10.1063/1.1698510View Description Hide Description
This paper introduces a method, based partially on existing non‐linear theory and partially on experimental results, for transforming a non‐linear oscillatory system into an equivalent linear one, regardless of the amount of non‐linearity in the original system.
20(1949); http://dx.doi.org/10.1063/1.1698511View Description Hide Description
Using a self‐biased electron gun the current arriving at the specimen, the current density at the specimen, and the area of illumination of the specimen have been determined for different values of the filament height. The variation of current density at the specimen with angular aperture of illumination has been determined and the results compared with those for the zero‐biased gun. The performance of the self‐biased gun is assessed and some of the factors involved in its operation and maintenance are discussed.
20(1949); http://dx.doi.org/10.1063/1.1698512View Description Hide Description
The effect of point material and point radius of curvature on positive and negative intermittent corona onset potentials has been studied with a point‐to‐plane gap in air at atmospheric pressure. The negative Trichel pulse onset strangely is independent of point material but does depend on point history and radius. This surprising result is shown to come from the circumstance that the Trichel pulse onset depends on current densities needed to condition the point surface and yield a higher value of the second Townsend coefficient to give the increased currents. Trichel pulse onset thus does not mark the onset of a self‐sustaining discharge. The self‐sustaining discharge initiates at lower potentials and leads to currents of a low order until the cathode spot cleans up.
20(1949); http://dx.doi.org/10.1063/1.1698513View Description Hide Description
Previous experiments have indicated that the magnetic permeability of sinterediron compacts is to a large extent determined by the final density of the compact. The permeability of five different ironpowders has proved to be independent of the origin of the powder, if the compacts were sintered at the same temperature. The results were compared with the theory of Polder and Van Santen and agreed very well with their predicted curve, if the residual pores were assumed to be flat disks. It could be expected that at higher sintering temperatures, the disk‐shaped pores would change into a more equiaxed or spherical shape.
Experiments to check this theory have been made, and the results are in good agreement. Compared for identical densities, the permeability of different ironpowders is appreciably higher if sintering is done at 1250–1350°C and for 24 hours, instead of at 1150°C for one hour. Different powders, however, now show quite different permeabilities, which could be explained by a different inclination of the powder to form spheroidal pores.
It is shown that coining after high temperature sintering brings the permeability values back to the curve of the disk‐shaped flat pores. The effect of the increase in permeability at higher temperature cannot, therefore, be due to a purification process, but must be attributed to a change in the pore shape.
20(1949); http://dx.doi.org/10.1063/1.1698514View Description Hide Description
Calculations are carried out which show that, because of the smallness of the cathode spot, the cathode is not able to dissipate the heat generated with heavy currents by conduction. Therefore, a much higher vaporization of cathode material per coulomb occurs for heavy currents than for low currents.
20(1949); http://dx.doi.org/10.1063/1.1698515View Description Hide Description
The present work contains an investigation of the e.m.f. method to determine the input impedance of a thin biconical antenna as first suggested by Schelkunoff. By considering the various components of the currents flowing on the antenna, it has been shown that the sinusoidal part is, indeed, the dominating component among all these currents. An investigation of the total current at the end of the antenna, where the lateral surface of the cone and the spherical cap meet, shows that the current is not identically zero at that point, but vanishes as the reciprocal of the square of the characteristic impedance of the cone. The two approximate expressions of the complex conjugate power used in the e.m.f. method are derived, as well as the exact expression. Computation of some integrals occurring in the formulation is also treated in detail to show the various approximations involved.
Errata: On Theoretical Signal‐to‐Noise Ratios in F‐M Receivers: A Comparison with Amplitude Modulation20(1949); http://dx.doi.org/10.1063/1.1698517View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698518View Description Hide Description