Index of content:
Volume 21, Issue 8, 01 August 1950
21(1950); http://dx.doi.org/10.1063/1.1699747View Description Hide Description
The conductivity of liquid selenium has been measured in the temperature range 200°–500°C. The resistivity was expressed by log10ρ=A+(B/T). Average values for different selenium lots and melts of A and B were −3.81 and 5850. The maximum deviations from the averages were 10 percent and 3.4 percent respectively. The resistivity was a function of temperature alone.
Various non‐metal impurities Cl2, I2, P lowered the resistivities and produced different values of B in different temperature ranges. Mercury addition caused no change in either A or B although that metal greatly influences the resistance of solid hexagonal selenium. Melts doped with Cl2 or Br2, along with Hg exhibited behaviors different from those with single additions.
It was concluded that selenium is an ideal semiconductor in the range of measurement.
21(1950); http://dx.doi.org/10.1063/1.1699748View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699749View Description Hide Description
The effects of inserting a video filter of finite width on the observability of pulsed signals in random noise are examined. It is found that at match (when the pulse and IF filter are each other's conjugate Fourier transforms) no improvement is gained from such a video: the infinitely wide response yields the optimum results. This is strictly true when the second detector is a quadratic rectifier; (a slight improvement on narrowing the video is noted, however, for strong signals when a half‐wave linear rectifier is used). Away from match a video filter does give noticeable improvement when the pulses are overlong, i.e., narrower spectrally than the IF (λ>1); more noise than signal is then removed by the narrower video filter. The greatest gain over the infinite video is observed for final filters slightly wider than the original pulse. On the other hand, for pulses that are too short (λ<1) the performance is worsened. The mean maximum signal level is decreased more rapidly than the interfering noise background. In all instances the familiar phenomenon of modulation suppression arises. Two different pulse shapes are considered: (1) Gaussian and (2) rectangular, but for wide videos pulse shape is not a critical factor. Curves showing the output signal‐to‐noise ratios as a function of the input ratio are included for conditions of match (λ=1) and mismatch (λ≠1).
21(1950); http://dx.doi.org/10.1063/1.1699750View Description Hide Description
The corona characteristics of air and methyl chloride were studied in a modified type of Cottrell precipitator at temperatures from room temperature to 500°C and pressures from 1 to 5 atmospheres. Within this range the corona current is a power function of voltage with gas density as a parameter. This power is independent of density. Values are given for air and methyl chloride.
21(1950); http://dx.doi.org/10.1063/1.1699751View Description Hide Description
General results obtained previously for slots of arbitrary shape are applied to the case of a transverse rectangular slot in a circular cylinder. It is shown that the principal transverse plane pattern of such a slot in which the excitation has only a circumferential tangential electric field component is the same as the pattern generated by an infinite axial slot with the same circumferential excitation. Theoretical and experimental curves are given for the narrow‐width half‐wave‐length slot.
21(1950); http://dx.doi.org/10.1063/1.1699752View Description Hide Description
The thermal conductivity of soils of varying moisture content is calculated on a physical basis. The results of this computation are, in a range of moisture contents from 5 to 25 percent, in agreement with experimental data in the literature. The influence of soil composition on conductivity is also explained by the theory. The use for practical purposes of an average resistivity value of 60 to 70 thermal ohms appears justified in the light of this analysis.
21(1950); http://dx.doi.org/10.1063/1.1699753View Description Hide Description
The differential equation is considered which determines the position of a vehicle from dynamical measurements of the non‐gravitational accelerationb made internally. Three linear approximations to the gravitational fieldg(r) of the earth, which lead to explicit solutions of this equation, are considered and their limitations are discussed. An interval‐wise solution (linear continuation) for trajectories of extended range is described, which is based on such linear approximations and has definite advantages in this application. The theory is applied to the trajectory of the German A10 vehicle.
21(1950); http://dx.doi.org/10.1063/1.1699754View Description Hide Description
Measurements were made of the diffraction patterns of circular apertures from one to eight wave‐lengths in diameter in the planes of the apertures and in the neighborhood of the apertures when a plane polarized electromagnetic wave was incident upon them. From Thomas Young's theory that the diffraction pattern is an interference pattern between the incident plane wave and wavelets from the edge of the aperture, the positions of maximum intensity have been predicted in the neighborhood of the aperture and the values of the intensities over the apertures checked with experiment.
21(1950); http://dx.doi.org/10.1063/1.1699755View Description Hide Description
The motion of the cathode spot in a direction opposite to that predicted by Ampère's law depends on the arc current, gas pressure, kind of gas, and magnetic field strength. Studies of the effects of these variables have been made, including measurements of velocity and the critical pressure at which reversal of motion occurs. The phenomena observed have not been clearly explained by any of the pictures presented to date. The existence of the retrograde motion indicates very strongly that the positive‐ion space charge outside the cathode is all important in determining the mechanism of current transfer.
21(1950); http://dx.doi.org/10.1063/1.1699756View Description Hide Description
Even under the assumptions of irrotational, isentropic flow, which have been found generally useful for subsonic and supersonic cases, the equations of gas flow are relatively intractable for mixed, transonic situations. Approximate methods of solution used by a number of investigators are reviewed briefly, as well as the hodograph technique, which yields exact solutions of the equations for plane flow. It is pointed out that all the methods predict smooth, potential mixed flows involving imbedded regions of supersonic speed and both acceleration and deceleration through the speed of sound. There is no experimental verification of the existence of such flows.
Three possible explanations for this sharp discrepancy between experiment and perfect‐fluid theory have been advanced; namely, (a) effects of viscosity, (b) non‐existence of neighboring solutions, and (c) temporal instability. These are reviewed in turn. None has led to a complete explanation, to date. Kuo's stability calculations are described briefly. His results indicate that stable, smooth, mixed flows may exist if certain conditions are satisfied.
Analysis of Intensities Obtainable Using Pinhole and Slit Collimators in X‐Ray Powder Diffraction Techniques21(1950); http://dx.doi.org/10.1063/1.1699757View Description Hide Description
Other factors being equal, the integrated intensity of a powder diffraction reflection is a function of the geometry of the source, collimator, and sample. By means of a geometrical and algebraic analysis one can predict the relative intensities to be expected with pinhole or slit collimators of different dimensions. Important gains in intensity can be obtained by replacing pinholes with slits, and the advantage is about the same if the x‐ray focal spot is viewed either longitudinally or laterally through slits of sufficient length. These results confirm experimental results reported by other investigators and also re‐emphasize the potential value of the ``side'' windows of x‐ray tubes, which are commonly neglected.
21(1950); http://dx.doi.org/10.1063/1.1699758View Description Hide Description
According to Guinier the low angle x‐ray scattering of particles should disappear when they are immersed in a fluid of equal electron density. This furnishes a way of measuring the electron density of powders, providing a suitable impregnating fluid can be found. Experiments of this type have been carried out with carbon black. A method of calculating the low angle scattering coefficient defined by Warren is given for impregnated samples. Measurements of the total low angle scattering intensity were made using a double crystal spectrometer and Geiger counter. No liquid was found that would give complete matching but approximate matching was obtained with phosphoric acid solution. The data extrapolate to show an electron density of 1.1N±0.1N where N=Avogadro's number. This compares with a value of 0.99N as calculated from the helium immersion mass density measurement on the same sample.
21(1950); http://dx.doi.org/10.1063/1.1699759View Description Hide Description
A removable intermediate lens is described which extends the range of magnification of a conventional instrument to 25:1 without sacrificing the accessibility of the objective and projection lens polepieces. The distortion, sensitivity to changes in accelerating potential and reproducibility of calibration are studied and found to be equal to, or better than in the standard arrangement. The system provides two values of the magnification which are essentially independent of the accelerating potential.
The Wall‐Quenching of Laminar Propane Flames as a Function of Pressure, Temperature, and Air‐Fuel Ratio21(1950); http://dx.doi.org/10.1063/1.1699760View Description Hide Description
The quenching distance between plane parallel plates has been measured for propane‐air flames at eight pressures ranging from 0.0832 to 2.77 atmos., and air‐propane ratios ranging from 11 to 24. Other tests have been made at atmospheric pressure in which (a) the temperature of the quenching plates has been varied from 80° to 715°F, (b) the temperatures both of the inlet gas and the quenching plates have been varied from 80° to 545°F, and (c) a series of six different solid surfaces was tested. The minimum quenching distance is found to occur at an air‐propane ratio between 13.5 and 14.0, and is proportional to the minus 0.91 power of pressure. Lean mixtures are found to be slightly less pressure‐sensitive. When both the inlet gas and plates are heated, the minimum quenching distance is found to be proportional to the minus 0.5 power of absolute temperature. The quenching effect appears to be independent of the nature of the surface.
21(1950); http://dx.doi.org/10.1063/1.1699761View Description Hide Description
Maxwell's equations are solved for a perfectly conducting conical wave guide and the propagation coefficients of this guide are discussed. The field at the mouth of a finite conical horn is estimated and the radiation therefrom is calculated in integral form. These integrals are solved in series form for horns of small flare angle and moderate length and the results compared with experiment.
21(1950); http://dx.doi.org/10.1063/1.1699762View Description Hide Description
Diffraction patterns of radar waves have been measured about the edge of a perfectly conducting wedge. Theoretical patterns have been calculated using an asymptotic solution suggested by Pauli. Good agreement is observed between experimental and calculated patterns. The thin wedge tested showed much similarity in diffracting properties to a suitable semi‐infinite conducting screen. The results may have application to diffraction effects about wedge‐shaped wings on high speed aircraft and missiles.
21(1950); http://dx.doi.org/10.1063/1.1699763View Description Hide Description
The solidification behavior of small metal (10 to 100 micron diameter) droplets has been observed on a high temperature microscope stage. An abrupt change in surface appearance and in the case of high melting metals a sudden brightening (``blick'') accompanies solidification. The solidification temperatures observed for a collection of droplets may be widely distributed, but a significant, usually the major, fraction of the droplets supercool some maximum amount (ΔT ‐)max that is reproducible and characteristic of the metal. For many metals (ΔT ‐)max≈0.18 times the absolute melting temperature. (ΔT ‐)max is not much changed by wide variations in the cooling rate and droplet size. The growth rate of metal crystals is very great so that the solidification rate of the droplets is controlled by the nucleation frequency.
Mass Spectrometric Study of Solids I. Preliminary Study of Sublimation Characteristics of Oxide Cathode Materials21(1950); http://dx.doi.org/10.1063/1.1699764View Description Hide Description
Mass spectrometric techniques have been utilized to study sublimationcharacteristics of materials used in electron tubes. Exploratory experiments and methods are described. Preliminary results concerning general processes induced by heat and by electron bombardment are discussed. Photographs are shown of mass spectrometer traces which demonstrate the effects described. Phenomena noted include (1) the inefficiency of electron bombardment for producing material transport from solids, (2) the thermal evaporation of alkali metal atoms and ions from bare nickel alloys, but no such evaporation from oxide cathodes formed on the same materials, (3) the complex thermal evaporation of oxide cathode constituents, (4) absence of positive ion evaporation from oxide cathodes, (5) the confirmation of mechanisms for negative ion formation in vacuum systems, and (6) the liberation of molecular oxygen from oxide cathodes which increases with electron emission drawn.
21(1950); http://dx.doi.org/10.1063/1.1699765View Description Hide Description
An extensive study of the charge and size distribution of particles from 0.5 to 30 microns radius in dust clouds of diverse exemplary types dispersed in air under a variety of conditions ranging from blowing with minimum of turbulence to conditions of violent and maximum impact with various types of surfaces was made using adaptations of the Hopper and Laby modification of the oil drop experiment. It was observed that all dusts, including homogeneous dusts with no impacts on solid surfaces, were charged. In homogeneous systems the charges of opposite sign were equal, no net charge resulting, the number of particles of similar size with opposite charges being about the same. The magnitude of charges increased somewhat less rapidly than the surface of the particle. There was no correlation between size and sign of charge. There is strong evidence that charging occurs on separation of the contacts between particles in the dispersion of the cloud. Humidity did not affect the charging. Studies of heterogeneous systems making contact with solid walls of different composition from the powder gave consistent asymmetry of charge of varying degrees depending on the proportion of particles striking the surfaces relative to those just separated.
21(1950); http://dx.doi.org/10.1063/1.1699766View Description Hide Description
The interaction of small particles suspended in air with the ions normally produced in the air is discussed. The neutralization of highly charged dust particles, if suspended in air, is treated in detail. The result is found to be in qualitative agreement with experiment. The special case of final equilibrium is investigated. It is found that multiply charged particles should be present in appreciable number in coarse aerosols at all times. This is confirmed by experiments both for initially charged quartz dust and for initially neutral ammonium chloride smoke. The general nature of an equilibrium charge distribution is presented. Charges up to ten electron units are found to be not uncommon.