Index of content:
Volume 23, Issue 7, 01 July 1952
23(1952); http://dx.doi.org/10.1063/1.1702286View Description Hide Description
Using a method which is the electromagnetic analog of the scattering matrix formalism in the theory of nuclear reactions, two ``sum rules'' are derived for the frequency independent coefficients occurring in the admittance matrix relating the currents and voltages in a loss‐free microwave junction. These are sums, respectively, over the various modes of the guides entering the junction, and over the modes of the cavity comprising the junction (suitably defined). The results are used to estimate the effect on the admittance matrix of the higher modes, both of the guides, and of the junction proper.
23(1952); http://dx.doi.org/10.1063/1.1702287View Description Hide Description
Experimental methods are given for determining the Q of both high and low Q resonant cavities at microwave frequencies. The emphasis is placed on the practical measurements necessary in determining the properties of ionized gases in the 3‐ and 10‐centimeter wavelength range.
Methods of Measuring the Properties of Ionized Gases at High Frequencies. II. Measurement of Electric Field23(1952); http://dx.doi.org/10.1063/1.1702288View Description Hide Description
23(1952); http://dx.doi.org/10.1063/1.1702289View Description Hide Description
The initial conduction interval in thyratrons, or time interval during which the major rise in current occurs, is shown to be dependent on two parameters. The first of these is dependent on tube construction and the second on circuit elements. It is shown that both factors are significant in operation of hydrogen thyratrons in millimicrosecond switching service.
On Diffusionless Transformation in Au‐Cd Single Crystals Containing 47.5 Atomic Percent Cadmium: Characteristics of Single‐Interface Transformation23(1952); http://dx.doi.org/10.1063/1.1702290View Description Hide Description
The kinetics of single‐interface motion in the diffusionless transformation of Au‐Cd single crystals containing 47.5 atomic percent Cd were studied. A diffusion‐controlled relaxation phenomenon was found with an activation energy of the order of 24,000 calories per mole. The two observed characteristics of diffusionless transformation of this alloy, that the transformation does not take place isothermally, and that the velocity of transformation on cooling is considerably and consistently larger than that on heating, can be successfully interpreted from the relaxation hypothesis.
Specimen Charging in the Electron Microscope and Some Observations on the Size of Polystyrene Latex Particles23(1952); http://dx.doi.org/10.1063/1.1702291View Description Hide Description
From observations in the shadow electron microscope and the standard electron microscope it is concluded that specimen charging does not significantly change the magnification of the latter, provided that the specimen is less than a few microns in thickness and the illuminating electron beam simultaneously strikes a nearby grounded conductor such as the supporting mesh.
The difference in size of shadowed and unshadowed Dow Latex 580G, lot 3584 reported by Kern and Kern (J. Appl. Phys. 21, 705 (1950)) is attributed to an added layer of material on the shadowed particles. Evidence in support of this view is presented, and the results of other workers are discussed in the light of it.
23(1952); http://dx.doi.org/10.1063/1.1702292View Description Hide Description
Slotted wave‐guide arrays feeding into parallel‐plate regions have been used in some high speed scanners. Parallel‐plate regions also have been used for the suppression of second‐order beams of high gain arrays.
A theoretical expression is derived for the conductance of a longitudinal shunt‐slot in a rectangular guide when the slot is radiating into a parallel‐plate region of arbitrary plate spacing. Some peculiarities of the theoretical results are discussed. There is good agreement between theory and experiment.
23(1952); http://dx.doi.org/10.1063/1.1702293View Description Hide Description
Unless the primary source of noise is one of those which is theoretically tractable, the statistical properties of the noise have to be inferred from samples. We have developed some criteria to aid in the decision whether a sample can reasonably be assumed to have come from a Gaussian noise with predetermined parameters.
23(1952); http://dx.doi.org/10.1063/1.1702294View Description Hide Description
The contribution of space charge to the admittance of a diode with a retarding field is calculated by computing the instantaneous electronic current induced in an external circuit. A discussion of the variation of the susceptance of a typical diode with frequency is given.
23(1952); http://dx.doi.org/10.1063/1.1702295View Description Hide Description
The transient creep of single crystals prepared from 99.999 percent zinc was studied at 35°C and at strain rates of the order of 10−5/min. The results are given by the relation: creep rate=at−n , where a and n are constants and t is the time after applying the load. This is of the form expected from Andrade's equation, but the exponent n had only approximately the value ⅔ found by Andrade and varied from experiment to experiment. An additional instantaneous plastic strain occurs on applying the stress at the start of a creep test. It was found to be proportional to the subsequent transient creep rate (as measured by a in the above equation) in a series of tests made on a given crystal. In addition to the process of recovery from strain‐hardening whereby a zinc crystal completely resoftens in a few days at 35°C, there appears to be a rapid, but very limited, stage of recovery, the effects of which quickly disappear upon further deformation. Some crystals exhibited a decrease in plasticity when allowed to rest for about two days at 35°C after a creep test. The development of this ``rest‐hardening'' is dependent to some extent on the occurrence of creep prior to the rest period. Zinc crystals were observed to contract with time after removing the load in a creep test. The extent of this contraction varied among specimens and among different tests made with the same specimen and was of the order of 10−5 cm/cm or less. This appears to be the first report of such an after effect in single crystals of a pure metal. The results are discussed in relation to the findings of other workers and are interpreted in terms of recent models of the metallic state.
23(1952); http://dx.doi.org/10.1063/1.1702296View Description Hide Description
The theoretical distribution of velocity across a liquid (Newtonian) between two coaxial cylinders and the velocity gradient at the inner cylinder is calculated in a new way. The equation used for calculating the coefficient of viscosity of the liquid is also given. The theory agrees with experiment for all values of θ0/φ0≤1/2 (where θ0 is the amplitude of deflection of the inner cylinder and φ0 is the amplitude of the oscillations inexorably imposed on the outer cylinder).
23(1952); http://dx.doi.org/10.1063/1.1702297View Description Hide Description
Measurements were made of the heat released during decay of the afterglow, the viscosity, and the free electron density in active nitrogen. Calorimetermeasurements of the heat released during decay of the afterglow amounted to as much as 22.3 calories per gram of the gas, corresponding to an excitation of one part in 350 of the molecules present to an energy of 9.6 electron volts. The viscosity of the active nitrogen was found to be the same as that of the unactivated nitrogen within one‐half of one percent. Ionization in a stream of active nitrogen is shown to be incidental rather than essential to the production of the Lewis‐Rayleigh afterglow. Microwave measurements of the free electron density in a stream of active nitrogen indicated one free electron for each 2.3×108 molecules in the stream. Contrary to Mitra's theory, probes in the stream were found to respond to carriers of negative sign rather than positive ions. Space discharges in an expanding stream of active nitrogen are attributed to free electrons moving with the stream. In the use of active nitrogen for studies of low density flow, by observation of the afterglow, disturbing effects will occur because of heating and ionization, but the disturbances appear not to be so large as to interfere with its use.
23(1952); http://dx.doi.org/10.1063/1.1702298View Description Hide Description
The simplified theory of the Mach‐Zehnder interferometer developed by the latter author is tested. The reduction in the visibility of fringes formed by the interferometer due to an extended area light source is shown experimentally. This reduction is shown to be caused by the superposition of two separate effects which are characteristic of the two angular coordinates of the unit vector pointing from the center of the collimating lens toward each point in the plane of the area‐type source. The relation of each of the effects to that coordinate characterizing it is demonstrated experimentally to agree with the theory. The result of combining these two effects from all of the points of an area‐type source is experimentally seen to cause the ``focusing'' of the visible fringes and to limit them to a finite region of the intersecting beams determined by the size and shape of the source.
23(1952); http://dx.doi.org/10.1063/1.1702299View Description Hide Description
A knowledge of the permittivity and loss tangent of snow and ice is essential in studying both radar echoes from snow‐covered terrain and the attenuation of microwave energy through snowstorms. To provide this information, a program was carried out at the National Research Council of Canada to measure the permittivity and loss tangent of ice and snow, and also to determine the reflection coefficients of snow‐covered surfaces, at a wavelength of 3.2 centimeters.
As a result of these investigations values have been obtained for the permittivity and loss tangent of snow of varying density, temperature, and water content. Theoretical values of the reflection coefficients of snow‐covered surfaces, calculated from these data, are compared with the values obtained from the direct measurement of reflections from natural snow surfaces.
23(1952); http://dx.doi.org/10.1063/1.1702300View Description Hide Description
The relativistic differential equations for the axial motion of an electron in a wave guide, excited by a progressive, sinusoidal, longitudinal, electric wave of constant speed less than that of light in vacuum, have been integrated. The first integral yields the results of J. C. Slater and gives the energy‐phase relationship, which is periodic. A second integration yields the phase distance relationship and thus completes the determination of the orbits. All separatrices are exactly integrable in terms of trigonometric and hyperbolic functions, while the general case requires elliptic integrals of the first and third kinds. The former are obtainable from tables, while the latter are evaluated by series expansions, or in the case of particles near the self‐crossing point of the separatrices where the convergence of the series expansion is slow, by the use of the addition theorem for elliptic integrals of the third kind.
These results are applied to the case of the proposed new Purdue Linear Accelerator. Energy distribution of the particles coming out of the end of the first two sections of this accelerator is in this way obtained.
23(1952); http://dx.doi.org/10.1063/1.1702301View Description Hide Description
The existing theories of the resistivity of mixtures assume regular arrangements of the two components, rather than random mixtures. A theory for a random mixture is given, based on the assumption that each crystal acts as if surrounded by a homogeneous medium whose properties are those of the mixture. Comparisons with experiment are made. The experimental data that have been examined fall roughly into two classes. One class consists of mixtures, where the variation of resistivity with composition disagrees violently with this theory, making it clear that the assumptions made are completely inapplicable. The remaining class consists of mixtures which generally agree well with the theory.
23(1952); http://dx.doi.org/10.1063/1.1702302View Description Hide Description
In continuation of a previous study of grain boundarydiffusion of silver in copper the grain boundarydiffusion of zinc along grain boundaries of columnar copper has been investigated at various temperatures. The results confirm the influence of the angle between the grains on diffusion and indicate an angular dependence of the activation energy. The latter reaches very small or even negative values in a certain range of orientation in agreement with the proposed model.
23(1952); http://dx.doi.org/10.1063/1.1702303View Description Hide Description
Radioactive tracers have been used in a study of evaporation of the alkaline earths, barium,strontium, and calcium from oxide coatings on passive nickel bases. Several percent of the total cathode Ba, Sr, and Ca leave a triple oxide cathode during processing and aging. The rates of transfer of these elements from the cathode during life tests of 118 hours at true cathode temperatures from 1175 to 1279°K obey equations of the form, logW=A−(B/T), where W is the rate of transfer in g/cm2/sec, T is the absolute temperature of the cathode,A and B are constants. The transfer of Ba from a BaO cathode obeys a similar law. The mass of Ba transferred in 118 hours of life from a triple oxide cathode at 1123°K true as 2.9×10−6g/cm2, and from a BaO cathode 1.4×10−5g/cm2. The masses of Sr and Ca transferred from a triple oxide cathode at the same temperature were 2.1×10−7 and 1.2×10−8g/cm2, respectively.
Bombardment of the anode by an electron beam during life tests decreased the quantity of Sr observed on the anode.Anodecurrent densities of 5 to 10 ma/cm2 at electron energies of approximately 100 volts produced a fiftyfold reduction in the quantity of Sr on the anode. Experiments indicated that this decrease was not due to heating of the anode. The quantity of Ba deposited on the anode was not decreased by electron bombardment.
The very considerable transfer of Ba from the cathode during processing was greatly reduced by breaking down the carbonates and high temperature flashing the cathodes in the presence of a few mm pressure of argon gas.
23(1952); http://dx.doi.org/10.1063/1.1702304View Description Hide Description
Conclusions important to the practice of electron microscopy are drawn from experiments. Particles of Dow Latex 580G are unstable under electron beams and best micrographed at low intensity. Two different effects of radiation are noted. At low intensity the mean diameter in the present work was 2890A, the standard deviation of the spherical particles was 60A. 580G is improved as an internal standard if recalibrated by some other suitable standard (not the reverse) each time before use. Fresh replicas of diffraction gratings are still the most practical standards, reproducing gratings to two percent. By an independent optical method (spectroscopy) one can measure the grating space of a replica when it is on an eighth‐inch, 200‐mesh specimen screen under exact calibration conditions for electron microscopes. Specific replicas, and from them specific preparations of 580G are checked easily and accurately by the spectroscopic method. Replica age or other factors which may affect accurate reproduction of an original grating are then no longer important in calibration. A reliable method of calibration using 580G is suggested.