Index of content:
Volume 21, Issue 7, 01 July 1950
21(1950); http://dx.doi.org/10.1063/1.1699720View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699721View Description Hide Description
Theory and design equations are presented for the conversion of a microwave, narrow‐band f‐m signal into very nearly a pure a‐m signal within an ordinary wave guide of specified dimensions. Frequency relationships and guide dimensions are arranged so that the f‐m side bands are shifted with respect to the carrier until they become a‐m side bands at the guide output. A complex intelligence signal may be transmitted with negligible distortion if it is used to amplitude modulate an intermediate carrier which frequency modulates the microwave carrier.
21(1950); http://dx.doi.org/10.1063/1.1699722View Description Hide Description
A new method dispensing with the use of radioactive tracers has been developed for measurement of volume self‐diffusion. The method is based upon the relationship of the rate of sintering of metallic wires to the coefficients of self‐diffusion. The values of the self‐diffusion coefficients for silver as measured by this method are in good agreement with those obtained by the radioactive tracers method.
A Graphical Analysis of the Interference Patterns of an Elevated Ultra‐High Frequency Antenna under Conditions of Atmospheric Stratification21(1950); http://dx.doi.org/10.1063/1.1699723View Description Hide Description
Analytical computation of the detailed lobe structure of an elevated meter or centimeter band antenna for a standard atmosphere is quite tedious. In case there are discontinuities in the gradient of refractive index the analysis becomes hopelessly involved.
This paper presents a straightforward method of graphical analysis for the optical region, equally applicable to a standard or stratified atmosphere.
21(1950); http://dx.doi.org/10.1063/1.1699724View Description Hide Description
Starting with the differential equation relating the output and the input of a linear varying‐parameter network, it is shown that the impulsive response of the system is related to a Green's function associated with the system through a linear operator which is the adjoint of the right‐hand operator in the given differential equation. A perturbation procedure for the determination of the impulsive response of a slowly varying network is outlined. Use of the method is illustrated by a simple example involving a band width‐modulated RC half‐section.
21(1950); http://dx.doi.org/10.1063/1.1699725View Description Hide Description
The theory of prediction described in this paper is essentially an extension of Wiener's theory. It differs from the latter in the following respects.
1. The signal (message) component of the given time series is assumed to consist of two parts, (a) a non‐random function of time which is representable as a polynomial of degree not greater than a specified number n and about which no information other than n is available; and (b) a stationary random function of time which is described statistically by a given correlation function. (In Wiener's theory, the signal may not contain a non‐random part except when such a part is a known function of time.)
2. The impulsive response of the predictor or, in other words, the weighting function used in the process of prediction is required to vanish outside of a specified time interval 0≤t≤T. (In Wiener's theoryT is assumed to be infinite.)
The theory developed in this paper is applicable to a broader and more practical class of problems than that covered in Wiener's theory. As in Wiener's theory, the determination of the optimum predictor reduces to the solution of an integral equation which, however, is a modified form of the Wiener‐Hopf equation. A simple method of solution of the equation is developed. This method can also be applied with advantage to the solution of the particular case considered by Wiener. The use of the theory is illustrated by several examples of practical interest.
21(1950); http://dx.doi.org/10.1063/1.1699726View Description Hide Description
A theoretical analysis of the effect of the spaces between drain tube units as used in the artificial drainage of soil is given. The problem is one of potential flow; therefore, the results are applicable to heat flow, etc. The basic problem solved is that for axially symmetric flow from an external cylindrical boundary at constant potential to a series of equal, equally spaced openings at a lower potential, all located axially on, and comprising a part of, the otherwise impervious drain tube. The radii of the open sections and impermeable sections of the drain tube are equal. The basic problem is extended to obtain the solution to the practical problem—the seepage of ground water into drain tubes beneath a horizontal water table. The exact solution of the basic problem is not suitable for numerical work. Accordingly, approximate solutions of specified uncertainty are derived and are utilized for tabulation of numerical results. As an example, the analysis shows, in the case of 6 in. diameter drain tubes having 1 ft. long impermeable sections and buried 4 ft. deep in uniformly permeable soil, that increasing the openings from in. width to ¼ in. width will increase the flow 36 percent; while embedding the tubes in gravel, to make the in. openings of effectively infinite width, will increase the flow 180 percent.
21(1950); http://dx.doi.org/10.1063/1.1699727View Description Hide Description
This paper presents the results of measurements of the signal strength and phase of 3.2‐centimeter radio waves for a 26.5‐mile path over the Gulf of Mexico under a variety of meteorological conditions. The radio data are used to determine the modified index‐of‐refraction as a function of height and the associated attenuation factor. The modified index‐of‐refraction curves are compared to those measured meteorologically.
21(1950); http://dx.doi.org/10.1063/1.1699728View Description Hide Description
A three‐dimensional array of thin conducting disks has found special application as an artificial refracting medium at microwave frequencies. Treatment of these refractive properties at oblique incidence requires specification of the dielectric and permeability coefficients of the medium.
When an alternating magnetic field is parallel to the disk faces, the field is undisturbed and the relative permeability coefficient is unity. When the alternating magnetic field is normal to the disk faces, circulating currents are induced on them. The boundary value problem of determining the current distribution on a single perfectly conducting disk is carried out in detail for the case where the disk diameter is small compared to the wave‐length. This current distribution is found to be representable by a magnetic dipole. If the disks in an array are far enough apart to neglect interaction, a simple summation of the dipole moments shows the array to have a diamagneticsusceptibility in the direction normal to the disk faces.
Combining this result with an expression for the dielectric coefficient, which was developed earlier by Kock, the constants of the anisotropic array are completely specified.
21(1950); http://dx.doi.org/10.1063/1.1699729View Description Hide Description
Mathematical methods of stress analysis are presented for linear, compressible, viscoelastic, or anelastic, materials such as metals at high temperatures or high polymers with small strains. For such materials stress, strain and their time derivatives of all orders are related by linear equations with coefficients which are material constants. Fourier integral methods are used to show that static elasticity solutions can be used to determine the time dependent stresses in viscoelastic bodies with any form of boundary conditions.
If stress and double refraction and their time derivatives are also linearly related, the standard photoelastic techniques can be used to determine the directions and difference in magnitude of the time dependent principal stresses, even though the principal stress axes do not coincide with the polarizing axes and both vary with time. When viscoelastic models are used in photoelastic studies, the time variation of the stress distribution in the model represents a first approximation to the dependence of the stress in the elastic prototype on Poisson's ratio.
21(1950); http://dx.doi.org/10.1063/1.1699730View Description Hide Description
In this paper it is shown how the low frequency index of refraction of a metallic delay‐lens medium may be calculated from electrolytic‐tank measurements on individual metallic elements of the medium. The proximity between adjacent elements is taken exactly into account for low frequencies. The test apparatus is described, and measured results are presented for three kinds of delay‐lens structures.
21(1950); http://dx.doi.org/10.1063/1.1699731View Description Hide Description
Experiments were conducted to study the role of the temperature of oxide‐coated cathodes in the glow discharge of argon, neon, and mixtures of neon and argon, and mercury vapor and argon.
It was found that for tubes without mercury present, in the region of cathode temperature between 300°K and 750°K: (1) The sparking potential falls with increasing cathode temperature. (2) The regulation voltage rises with increasing cathode temperature. For argon and mercury vapor, the regulation voltage falls with increasing cathode temperature. The action here is attributed to metastable states of inert gas. (3) The current density in the glow discharge decreases with increasing cathode temperature. This, too, is attributed to the action of metastable states. (4) The minimum sustaining current in a glow discharge is found to decrease with increasing cathode temperature and rise with increasing pressure. (5) The current density in the glow discharge varies roughly linearly with pressure, or at a slightly higher rate, in contrast with the space‐charge derivations which claim a variation with pressure squared. (6) The regulation voltage tends to rise very slightly at lower pressures.
21(1950); http://dx.doi.org/10.1063/1.1699732View Description Hide Description
An approximate method for estimating radiant heat transfer from gaseous emitters is proposed. An average absorption coefficient is used for the effective width of an entire vibration‐rotation band. The procedure for determining an average absorption coefficient in terms of the integrated absorption is justified, approximately, for very small optical densities and also for large total pressures where the spectral half‐width is no longer small compared with the rotational spacing. Because of these limitations it is to be expected that the procedure proposed here will be particularly useful only in estimating gaseous emissivities for emitters in high‐pressure combustion chambers. A preliminary compilation is given of the theoretical results which are necessary for making rapid estimates of the emissivities, over a wide temperature range, for CO, HI, HBr, HCl, and HF.
21(1950); http://dx.doi.org/10.1063/1.1699733View Description Hide Description
The long range order in β‐Brass has been determined by measuring the integrated intensity of the (100) superstructure reflection from a single crystal held at various temperatures. With a single crystal, and CuKα‐radiation obtained by a balanced Ni‐Co filter, there is ample intensity for measurement with a Geiger counter spectrometer. The measured long range order parameter S is in satisfactory agreement with the theoretical predictions.
21(1950); http://dx.doi.org/10.1063/1.1699734View Description Hide Description
A formula for determining the resolving power of an x‐ray microscope built according to the design suggested by Kirkpatrick and Baez is derived, and an estimate of that resolving power given. The optimum resolving power is such as to resolve points separated by a distance of the order of 2000A.
21(1950); http://dx.doi.org/10.1063/1.1699735View Description Hide Description
Tobacco mosaic virus has been used as a known test object for the intercomparison of several methods of small particle measurements, all involving metallic evaporation. Various characteristics of these methods, which determine their accuracy, were investigated. Measurement of groups and the derivation of a mean size therefrom is primarily limited in accuracy only by the exactness of juxtaposition of the successive particles in the group. The measurement of the images of single particles is subject to the relatively large error associated with the uncertainty in the relationship between the true particle edge and the image of the edge which has received a coating of metal. The measurement of shadow lengths reduces this error by the factor 2 tanα/2. A symmetrical shadow method is described which reduces the error of substrate tilt, and is insensitive to amount of metal deposited. An average over the four best methods gave a value 150.0±3.6A agreeing closely with the x‐ray determination.
21(1950); http://dx.doi.org/10.1063/1.1699736View Description Hide Description
The image formed in the electron microscope of shadowed polystyrene latex spheres is larger than the image of unshadowed spheres. This increase in image size and probable causes are discussed. Implications of this phenomena on the study of viruses and other biological specimens are mentioned.
21(1950); http://dx.doi.org/10.1063/1.1699737View Description Hide Description
A hysteresis in the rectification of the contact aluminum‐aluminum oxide‐electrolytic solution was found by Dekker and van Geel. In the present paper, measurements of the current‐voltage characteristics between 1 and 2000 cycles are described. A tentative explanation of the hysteresis is given in terms of a pre‐breakdown phenomenon.
21(1950); http://dx.doi.org/10.1063/1.1699738View Description Hide Description
A powerful new scheme is presented for analyzing capillary flowmeasurements on non‐Newtonian liquids. The basic disability of the capillary method, namely its inability to assign to a flow measurement any particular shear rate, or even a known range of shear rates, is overcome. This is accomplished by developing simple relations connecting the directly measured quantities, pressure and flow rate, with viscosity and shear rate. Application to Bingham bodies is easy and an illustration is given. Application to measurement at high rates of shear is immediate in principle, but there are certain allied measuring difficulties. These are discussed briefly. Finally, a simple approximate formula applicable to cases of moderate deviation from Newtonian behavior is given.
21(1950); http://dx.doi.org/10.1063/1.1699739View Description Hide Description
This paper presents a study of the sequence of events by which cracks ``grow.'' A careful investigation was made of the origin, development, and characteristic pattern of level‐difference markings left by rapidly moving fractures in a wide variety of materials.Characteristic markings indicating discontinuous propagation are found in fractures of plastics, polycrystalline and single‐crystal materials,coal, mica, photographic film from which the emulsion was stripped, etc. Thus, the continued occurrence of essentially discontinuous initiations which comprise fracturing becomes clear. These considerations are sufficient to explain a number of well known fracture markings and to clarify what is mean by brittleness and fracture velocity.