Volume 24, Issue 5, 01 May 1953

Arc Drop and Deionization Time in Inert Gas Thyratrons
View Description Hide DescriptionThe energy balance is discussed and a semi‐empirical equation for recovery time is derived; the discussion is based on diffusion alone without consideration of cumulative ionization and metastable states. Corroborating the theoretical considerations, measurements were made of arc drop and recovery time in thyratrons filled with mixtures of Xe with Ar, Ne, and He. The spectral emission lines in these discharges were photographed in the visible range. It was found that the arc drop remained fairly constant down to low concentrations of Xe and then increased steeply. Lines of the gas other than Xe also appeared only at low concentrations of Xe. The recovery time is a monotone declining function of the mixture of the heavy and the light components. The spectral lines are tabulated.

Pore Distribution in Porous Media
View Description Hide DescriptionAn attempt is made to determine the actual distribution of the pore sizes in a porous rock from the cumulative distribution of mercury injected into the rock with increasing pressure against capillary forces. The aim is to correct for those pores which could fill at a given pressure but which do not, because they are connected to the mercury source only by smaller pores. The presented correction is based on probability concepts.

Evaporation of Germanium Films from a Carbon Crucible
View Description Hide DescriptionEquipment for evaporation of germanium in vacuum at various measurable rates is described. The activation energy of evaporation is 3.5 ev, which is in close agreement with recent data by Searcy.

Propagation of Acoustic Waves in a Liquid‐Filled Cylindrical Hole, Surrounded by an Elastic Solid
View Description Hide DescriptionThe theory of elastic wave propagation in systems where discontinuities in the physical properties occur along concentric cylindrical surfaces is developed for an impulsive ring‐shaped source. This source is mathematically similar to the point source discussed by Lamb and Pekeris. The field problem is solved for a liquid‐filled column, surrounded by an elastic solid, representing a basic model applicable to well‐logging problems.

Subharmonic Oscillations in Nonlinear Systems
View Description Hide DescriptionThis paper deals with the subharmonic oscillations which occur in systems with nonlinear restoring force. It is first shown that the order of the subharmonics has a close connection to the form of the nonlinear characteristics. The subharmonic oscillation of order ⅓, i.e., the oscillation whose fundamental frequency is one‐third that of the applied force, is then particularly investigated for the cases in which the nonlinear characteristics are expressed by (1) cubic and (2) quintic functions. In both cases the stability problem of the periodic solution is discussed in detail according to the stability criterion given previously by the author. The analysis reveals that in the latter case (2) the second higher‐harmonic of the subharmonic, i.e., the oscillation of order ⅔ builds up with negative damping. This is verified by experiments in which the oscillation of order ⅔ causes the collapse of the original subharmonic oscillation.

Null Characteristics of the Rotating Adcock Antenna System
View Description Hide DescriptionIt has been the usual practice when designing a rotating Adcock antenna system to allow a spacing of one‐half wavelength between opposite dipoles at the highest intended frequency of operation. A mathematical analysis of this antenna system is presented, and it is shown that an optimum spacing for the most clearly defined nonmultiple nulls approaches a full wavelength. Useful design formulas are developed and typical response curves are presented graphically.

A Method for the Quantitative Determination of Preferred Orientation
View Description Hide DescriptionA new method for the study of preferred orientation in polycrystalline materials with a Geiger counter x‐ray spectrometer is described. The unique feature of the method is that a spherical diffraction specimen is employed. Sufficient data for the construction of a complete pole figure may be obtained, and the observed intensities may be plotted directly on a stereographic net without geometry or absorption corrections. The method may be used satisfactorily with the heavy metals since x‐rays are not transmitted through the specimen. A technique for accurately machining and electroetching spherical specimens is given, and the special goniometer used with the spectrometer is described. Sample data are given and their interpretation discussed. The method is compared with other recently developed techniques for the study of preferred orientation.

Free Space Radiation Impedance of Rhombic Antenna
View Description Hide DescriptionThe expression for the driving point impedance of a generalized electric circuit as vizualized by the author is partially integrated, and the physical significance of certain terms is discussed in connection with their application to antenna problems. Upon postulating an unattenuated traveling wave as a first approximation to the current along either a terminated rhombic or Vee antenna, a formula is derived for the free space radiation impedance of each. The expression radiation impedance is used for that portion of the driving point impedance which may be determined from the complex power into the external fields. The resistive component of the impedance for the rhombic antenna checks with the radiation resistance as listed by Leonard Lewin in a discussion of a paper by Donald Foster, with Lewin apparently obtaining his formula by the solid angle Poynting vector method.

The Effect of a Soller Slit on the Diffraction of X‐Rays by Deformed Crystals
View Description Hide DescriptionIn the application of the Guinier‐Tennevin focusing Laue method to the study of crystal distortion, it is found that the use of a Soller slit in the incident beam produces striations in reflections from certain crystals. The diffraction geometry of a beam so modified is examined in detail, and it is shown that the striations are due to torsion of the crystal. The relation between the amount of torsion and the slope and spacing of the striations is treated quantitatively. There results a useful extension of the focusing Laue method by which not only the bending, but also the torsion, of a crystal may be measured.

Current Element Near the Edge of a Conducting Half‐Plane
View Description Hide DescriptionThe two‐dimensional problem of the Maxwellian field from a line source of current adjacent to a conducting half‐plane is treated by a transform method of solution. Expressions in the form of contour integrals are given for the current induced in the conductor and for the radiated field.

Radiation Field of a Conical Helix
View Description Hide DescriptionIt is now well known that a cylindrical helix, when excited at frequencies corresponding to wavelengths comparable to the length of one turn of the helix, can radiate a sharp beam along the axis over a wide frequency range (about one octave). It is shown in the present communication that if the helix be conical instead of cylindrical (the diameter varying along the length of helix), then the axial mode of radiation can be maintained over a much wider band of frequencies. The radiation pattern of a conical helix, 60 cm diameter at the base, tapering linearly to 20 cm at the top in 10 turns within a height of 112 cm (with the ``ground'' provided by brass disk of 100 cm in diameter) has been studied experimentally. It is found that the axial mode of radiation is maintained from 150 Mc/sec to 450 Mc/sec. By increasing the number of turns, the band width can be considerably increased. Assuming a linear current distribution, theoretical expressions have also been deduced for E _{φ} and E _{θ} for a conical helix. Some modifications of the simple conical helix, such as may have special applications, are indicated.

The Effect of Pressure on the Tensile Properties of Several Metals and Other Materials
View Description Hide DescriptionTension tests to fracture under superposed hydrostatic pressure reaching to nearly 30 000 kg/cm^{2} have been conducted on five polycrystallinemetals,Ni,Ta, Cb, Mo, and W, two brittle single metallic crystals, Sb and Cu_{5}Zn_{8}, and two brittleamorphousmaterials, ``Melmac 404'' and B_{2}O_{3}glass. All these materials exhibit ductility at the highest pressure; the largest increases are for the polycrystallinemetals of which the largest is W, which breaks brittely at atmospheric pressure but with 81 percent reduction of area at the highest pressure. The shape of the strain hardening curves varies among the five metals from abrupt rise with long tailing off to long gradual rise with comparatively abrupt falling off, in the order W, Mo,Ta, Cb, and Ni. The character of the fracture varies continuously with pressure and is different for the different metals. Of the brittle single crystals Sb has a reduction of area of 55 percent at the highest pressure. The plastic, Melmac 404, gives the smallest increase of ductility, a reduction of area of only 14 percent at the maximum. This small value is doubtless associated with the absence of internal crystal slip planes. B_{2}O_{3}glass gave a reduction of 87 percent; this is largely a phenomenon of plasticity, but there is an appreciable component of viscosity. Both amorphousmaterials showed an increase of density after pulling under pressure instead of a decrease.

The Field Emitter: Fabrication, Electron Microscopy, and Electric Field Calculations
View Description Hide DescriptionWhen a specially designed field cathode assembly was inserted in a commercial electron microscope which was appropriately modified for the purpose, the geometry of the needle shaped emitter was obtained from micrographs of several of its profiles at various stages of emitter fabrication and experimental use. An investigation of several methods of fabrication revealed that a smooth, simple, and relatively stable tungsten emitter geometry resulted from a refinement of the methods of Benjamin and Jenkins which combines the electrolyticetch and the smoothing action of surface migration. The electric field at any point on an emitter surface was calculated when the emitter geometry was precisely fitted with one equipotential surface from the family surrounding a charged, isolated sphere‐on‐orthogonal‐cone. A theoretical surface distribution of current density for a typical emitter was derived from the calculated field distribution and the wave‐mechanical field emission theory. From this result was calculated a value of the emitting area which was in good agreement with experiment.

Elasto‐Plastic Stress‐Optical Effect in Silver Chloride Single Crystals
View Description Hide DescriptionThe use of silver chloride as a material for ``photoelastic'' stress analysis offers the possibilities of studying both elastic and plastic states of stress in a crystalline material on either a micro‐ or macroscale. It is necessary, however, to relate quantitatively the stress state and the observed relative retardation and extinction angle. In this paper these relationships are developed from a general theory of stress birefringence, according to a stress‐dependent hypothesis. This hypothesis and the resulting analytical relationships are experimentally vindicated by measurements made on a variety of single crystal specimens of silver chloride tested in simple tension in the elastic and plastic stress ranges.

The Gas Flow Past Slender Bodies
View Description Hide DescriptionThe following relates to moving objects immersed in a perfect gas otherwise at rest. The momentum of the resulting gas flow is defined. It is shown that it ultimately approaches a definite magnitude as the object assumes a steady motion, said magnitude being independent of the history of the previous motion.
This result is applied to the two‐dimensional flows of airship theory, and thus the airship theory is extended to include compressibility effects to a limited extent.

Space Charge Spread of Reflected Electron Beams Studied by a Photographic Method
View Description Hide DescriptionThe spread of electron beams of rectangular cross section is studied (a) in a field free space, (b) in a reflection space. An experimental method is tried to trace the electron paths from the excited gas molecules left behind by directly photographing the beam in a high vacuum. The results are checked with published theoretical calculations in case (a), and with results obtained through an iterative method in case (b).

Metal Capillary Cathodes
View Description Hide DescriptionSince 1941 we have been working on the development of new types of cathodes, using elements such as barium or thorium as emitters. If provided with a suitable base, these elements are able to migrate and, acting as an atomic layer, reduce the work function of the base material. By introducing a separate supply source below the emitting surface, a work‐function‐reducing substance will be caused to migrate to the cathode surface through small capillaries. Evidently a wide range of new types of cathodes is feasible, since the independently stored emitter of a metal capillary cathode can be held at a temperature different from that of the cathode surface. The subject of this paper is the description of some of these cathodes.

Modes in Wave Guides Containing Ferrites
View Description Hide DescriptionWhen a static magnetic field is applied in the longitudinal direction to a wave guide containing a ferritematerial, the medium within the guide becomes anisotropic. It has been shown by Polder that when a microwave field is propagating within such a medium, the relation between the components of B and H is given by: , where μ and μ′ depend on the static magnetic field. Using these equations, an investigation of the wave‐guide modes is made. Formal solutions are obtained for guides having circular symmetry. These solutions depend on the solution of some rather involved eigenvalue equations which determine the propagation constants, and which probably can be solved only by numerical methods.

Response of Linear Time‐Dependent Systems to Random Inputs
View Description Hide DescriptionA nonstatistical differential equation is derived for the mean square response of linear systems to random inputs. This equation can be used to determine analytically the steady state and transient response of systems with constant coefficients. It is also applicable to time dependent systems. While the solution cannot in general be obtained analytically in this case, the equation is of the type that can be immediately mechanized on analog computers.

Temperature Dependence of Low Frequency Fluctuations in Thermionic Emitters
View Description Hide DescriptionA method is described for measuring the temperature variation of the low frequency fluctuations of emission current (flicker effect) in diodes with oxide‐coated cathodes. In order to vary the cathode temperature and at the same time avoid the effects of space charge and the variation due to changes of the mean current, it is necessary to operate the test diodes under retarding field conditions. The results show that the majority of tubes exhibit minimum fluctuations at a cathode temperature in the region of 1000°K, although both rising and falling temperature characteristics are encountered. It is concluded that the flicker effect is in fact a combination of at least two separate phenomena. Finally, the various theories are examined in the light of the new information, and none is found to be completely satisfactory.