Index of content:
Volume 20, Issue 8, 01 August 1949
20(1949); http://dx.doi.org/10.1063/1.1698519View Description Hide Description
The Precision Determination of Lattice Constants by the Powder and Rotating Crystal Methods and Applications20(1949); http://dx.doi.org/10.1063/1.1698520View Description Hide Description
A method that makes possible the attainment of a very high degree of precision in the determination of lattice constants (in some cases more than 1:200,000, even with crystals of lower symmetry) is described. The method and its applications, which has been in steady development since 1934 (in Riga, University of Latvia), tries to eliminate the errors of the Debye‐Scherrer‐Hull and rotating crystal methods by a careful experimental technique, based on:
(1) Very exactly built cameras of a reasonable diameter (64 mm);
(2) Elimination of all well‐known errors of the powder and rotating crystal methods (e.g., elimination of film shrinkage errors by a new method of putting the film into the camera);
(3) Locating the camera in a specially constructed thermostat so that the temperature some hours before and during exposure is kept constant.
The films obtained in this way are very clear, especially in the important back‐reflection region, with distinct lines (or spots) that can be measured exactly. As no standard substances are necessary and the absorption corrections are negligible (using very thin samples), the method must be regarded absolute. Of course, the sample (crystal) must be prepared, mounted, and centered under the microscope, a procedure that is not difficult.
Applications: lattice constants and expansion coefficients (even within the range of 10°C); quantitative solubility determinations; appearance of new phases and transition points; the determination of the exact value of Avogadro's number; molecular and atomic weight determinations (if the exact density values are known); statement of imperfections in crystals.
20(1949); http://dx.doi.org/10.1063/1.1698521View Description Hide Description
The satisfactory use of Geiger‐counter methods for the measurement of diffracted x‐ray intensities demands that the observed counting rates be corrected for the non‐linear response of the counter. Both the multiple foil method of calibrating the non‐linearity of response and the two‐source method of Beers for measuring counter dead time have been found to be unsatisfactory approaches to this problem. The experience of this laboratory indicates that electronically controlled oscillographic techniques for the direct measurement of dead time are much more satisfactory. With a knowledge of the dead time, τ, the true counting rate, N, can be calculated from the observed counting rate, No , by employing the formula N=No /(1−No τ).
For a pulsating x‐ray source the effective counting rate is faster than the observed by a factor which is characteristic of the type of rectification employed, the target material, and the voltage applied to the x‐ray tube. A sufficiently accurate value of this factor can be arrived at by oscillographic observation of the pulse pattern and in some cases by mathematical analysis.
20(1949); http://dx.doi.org/10.1063/1.1698522View Description Hide Description
A rapid and accurate method of determining preferred orientation in the surface of thick samples was devised using a reflection technique on the Geiger‐counter spectrometer. By presenting the specimen face to all possible angles with respect to the incident x‐ray beam, through use of a special fixture, and recording the intensity of diffraction on a potentiometer recorder, the relative number of planes of a given set at each orientation can be determined. The change of intensity due to the positioning of the specimen face away from the Bragg angle is taken care of by a correction formula which is experimentally verified. The method is especially valuable for dealing with thin layers on thick specimens.
20(1949); http://dx.doi.org/10.1063/1.1698523View Description Hide Description
In order to gain additional information about the origin of preferred orientation of recrystallized grains, the influence of a magnetic field on recrystallization of iron‐cobalt alloys is being studied. Preliminary results indicate a change of the texture which can be qualitatively accounted for by considering the magnetostrictive properties of the alloy and their influence on the stability of the recrystallization centers.
20(1949); http://dx.doi.org/10.1063/1.1698524View Description Hide Description
Observations are given which favor a reticulate chain structure for carbon black. The discovery of ``hollow'' particles in carbon black dispersions of P‐33 and Thermax is reported along with possible explanations for the observed image intensities which give rise to the interpretation of hollowness. Striated surfaces strongly suggestive of a laminar structure and serrated edges which conform in orientation with these plateaus have been detected. Rough areas arranged both in lines and at random are observed over the surfaces of some particles. Shape variations away from the spheroidal and the occurrence of straight‐edged images are stressed again.
20(1949); http://dx.doi.org/10.1063/1.1698525View Description Hide Description
In an investigation of the coexistence relationships of n+1 phases in the isothermic‐isobaric sections of n‐component systems, Paul A. Beck found that there are at most two coexistence patterns possible. Furthermore, theoretically, only a well‐chosen sample need be examined for the phases it contains in order to clarify the phase coexistence relationships of the n+1 phases whose compositions are already known. The present paper gives formulas for calculating the composition of the ``well‐chosen sample.'' Supplementary formulas are given for the range of coexistence, in the composition space, for any n phases known to coexist.
20(1949); http://dx.doi.org/10.1063/1.1698526View Description Hide Description
Following a discussion of several methods of attack and their application to various particular cases, the problem of the diffraction of an electromagnetic wave through a plane, infinitely thin, perfectly conducting, perforated screen is formulated (in generalized, cylindrical coordinates) in terms of the (generalized Fourier) transform of the tangential electric field in the aperture. The result is an integral equation for this transform, which may also be expressed as an integral equation for the aperture field. The power transferred through the aperture is calculated and cast in a variational form of the Schwinger type, the real and imaginary parts of the reciprocal of the complex power being stationary with respect to first‐order variations about the real and imaginary parts of the exact aperture field. An aperture impedance is defined, whose real part (aperture resistance) is the ratio of the aperture scattering cross section to the geometrical cross section, and whose imaginary part is a measure of the standing waves in the neighborhood of the aperture. Moreover, the ratio of the aperture resistance to the impedance of the incident wave is equal to the ratio of the actual power transfer to that predicted by geometrical optics. The real (conductance) and imaginary (susceptance) parts of the aperture admittance are stationary with respect to first‐order variations about the real and imaginary parts of the exact aperture field, the former being an absolute minimum in all cases, and the latter only in special cases. A complementary formulation, more suitable to the scattering due to a disk of finite area, is given in terms of the current flowing in the screen. An obstacle admittance, analogous to the aperture impedance, is developed. The two formulations are related by a rigorous form of Babinet's principle (due to Booker, Schwinger, and others). Two more alternative formulations, developed by Copson, are cited. Explicit formulations are given in two‐dimensional Cartesian coordinates and in cylindrical polar coordinates. The results are applied to the diffraction of a plane wave through an infinite slit, where the magnetic field is parallel to the slit, and the diffraction of a normally incident plane wave through a circular aperture. The results for the slit compare favorably with the rigorous results computed by Morse and Rubenstein, while the results for the circular hole agree with those of Rayleigh and Bethe in the limit of large wave‐length, and with geometrical optics in the limit of small wave‐length. The Kirchhoff theory is developed in terms of the aperture conductance and compared with the more exact results. It is found to be very poor in the limit of large wave‐length (where the ``static'' methods developed by Rayleigh are valid) and satisfactory in the limit of small wave‐length (where geometrical optics give the transmission factor). The variational formulation provides a convenient continuation between the static and geometric limits and appears to be superior to the Kirchhoff theory for any presumed aperture field.
20(1949); http://dx.doi.org/10.1063/1.1698527View Description Hide Description
A beam of ultrasonic sound waves is projected across an air stream the velocity and temperature of which are required. From the known ultrasonic frequency and the measured wave‐length, the temperature may be determined; and from the slope of the beam boundary, an approximate Mach number may be obtained. The method is applicable to steady‐flow problems as in wind tunnels or to transient‐flow problems such as the study of shock waves.
20(1949); http://dx.doi.org/10.1063/1.1698528View Description Hide Description
Oxide cathodes prepared on a Si‐Ni alloy base metal have an interface of barium orthosilicate. The thickness of this layer is measured by means of an x‐ray method and found to increase with the life of the cathode and to be of the order of 10−3 cm. Measurements of the effective, specific electrical conductivity were made and compared with the conductivity of the coating, (BaSr)O. Both materials exhibit a conductivity‐temperature variation characteristic of semiconductors; however, the conductivity of the interface was always less than that of the coating. The interface layer influences the thermionic emissioncharacteristics of the cathode due to an interface voltage developed by the flow of emission current. A retarding‐potential method is developed for determining this voltage.
20(1949); http://dx.doi.org/10.1063/1.1698529View Description Hide Description
Fluid flow within the streamline (viscous) range, made visible by appropriate means, may be made to simulate fields occurring in electrostatic, magnetic, electric current, heat flow, chemical diffusion, and other situations. Simple, reliable, and inexpensive techniques for setting up the desired fluid flow situations have been worked out, and are described herein.
Two‐dimensional fields with one, several, or many non‐distributed sources (or sinks) may usually be made up and operated with ease. Techniques for the simulation of certain three‐dimensional fields with axes of symmetry have also been developed.
Through the invention of the ``sandbed'' feature, the fluid flow method has been greatly extended: fields due to distributed sources can be simulated, not only outside the sources, but inside them as well.
Numerous photographs of fluid mappers in operation, illustrating all of the foregoing types of cases, are included.
20(1949); http://dx.doi.org/10.1063/1.1698530View Description Hide Description
Rectifying current‐voltage characteristics going up to several hundred volts inverse have been observed in metal‐germanium point contact rectifiers. A reproducible negative differential resistance region occurs in the inverse characteristic. Certain impurities are desirable in producing high voltage material. Surface treatment, e.g., by etching, is very important. The metal used as a whisker has little effect. Increasing the force of contact increases greatly the current of low voltages but has less effect on the high voltage curve. Pronounced improvement of rectification can be effected by treatment of the contact with large currents.
Variation with temperature is very marked, especially for crystals of large inverse resistance; the variation of the inverse peak with temperature indicates that contact heating is responsible for the negative resistance. Time lags in the inverse negative resistance region of the order of 10−5 second occur.
When contact is made between two Ge crystals, typical inverse characteristics are observed in both directions. Photoelectric effects are observed and indicate that the barrier thickness is greater the higher the inverse peak voltage.
20(1949); http://dx.doi.org/10.1063/1.1698533View Description Hide Description
Measurement of the Dielectric Constant and Loss of Solids and Liquids by a Cavity Perturbation Method20(1949); http://dx.doi.org/10.1063/1.1698535View Description Hide Description
20(1949); http://dx.doi.org/10.1063/1.1698536View Description Hide Description