Volume 21, Issue 12, 01 December 1950
Index of content:

Electron Microscopy of Wet Biological Tissues by Replica Techniques
View Description Hide DescriptionThe study of wet biological tissuesin situ using the electron microscope was made possible by modifying existing replica techniques and developing a new wet replication method. The shortcomings of existing replica techniques are discussed and the modifications necessitated are described.
Replicas of wet tissues were produced in extremely thin polystyrene films which could be observed directly in the electron microscope; however, superior results were obtained by preparing silica replicas from the replicated polystyrene films. A modified two‐replica silica method proved satisfactory but the three‐replica silica method developed was preferable. Electron micrographs of wet wood fiber surfaces used to illustrate these techniques reveal new data concerning submicroscopic cell wall structure.
The problem of interpreting replicas of partially degraded and disorganized materials is illustrated and discussed briefly.

Magneto‐Ionic Triple Splitting of Ionospheric Waves
View Description Hide DescriptionIn recent years reports have appeared from various ionospheric observatories concerning the appearance of a third magneto‐ionic component or a so‐called z trace. The phenomenon has also been called magneto‐ionic triple splitting. After having recorded a number of such triple splits at the new Kiruna observatory, we became very interested in the problem as a whole. The entire matter was examined theoretically and as a result it has been possible to show that strong coupling often exists between the ordinary and z components as a critical level. It has further been possible to show that the z wave becomes strongly excited when the collisional frequency v approaches the critical collisional frequency v_{c} of Appleton and Builder. When v is larger than v_{c} the o echo rapidly disappears and only the z and x components remain (if v_{c} is sufficiently small, i.e. at very high latitudes). A useful expression for the transmission coefficient of the o —z wave has been derived.

The Equivalent Circuit for the Transmission of Plane Elastic Waves through a Plate at Oblique Incidence
View Description Hide DescriptionEquations are developed for the transmission of a plane elastic wave through a plate at oblique incidence with unequal fluid medium on each side of the plate. The plate transmitting both the dilatation and shear wave is reduced to a simple equivalent circuit with impedances which are functions of the incidence angle.

Gas Evolution in Liquids and Cavitation
View Description Hide DescriptionNine heavy lubricating oils, four light lubricating oils, three aircraft engine fuels, one diesel fuel, and distilled water were tested for their air solubility and for the rate of solution and evolution of air when equilibrium is disturbed. The technique consisted of shaking of a container partly filled with liquid and taking accurate pressure measurements of the air above the liquid. Since air in solution exerts no pressure, the variation of pressure during and after the agitation furnished information on the amount of air that went into or came out of solution and their time rates. The solution was considered to have reached equilibrium when further shaking no longer changed the air content of the liquid.
A wide variation exists in the solubility constants and rates, but the rate of evolution was always found to be proportional to the supersaturation, and the rate of solution to the undersaturation, under otherwise similar conditions. At room temperature the air solubility constant for distilled water is 1.84 percent, for a gasoline approximately 20 percent, and for (light and heavy) lubricating oils approximately 10 percent. The ``half‐life'' for evolution on the other hand was 3.86 sec. for distilled water, 0.13–0.24 sec. for gasoline, 3.6–7.6 sec. for light, and 11.6–51.4 sec. for heavy lubricating oils.
The investigation gave some insight into the phenomenon of cavitation which is still largely shrouded in mystery.

Electrical Conductivity Method for Measuring Self‐Diffusion of Metals
View Description Hide DescriptionA method has been proposed to measure self‐diffusion in metals by measurement of the contact conductance of an interface between two metallic spheres or hemispheres. By combining Kuczynski's equation for self‐diffusion with Bowden and Tabor's equation for the contact conductance between two metallic spheres a relationship has been obtained between contact conductance and the time of heating of the spheres. Thus by measurement of junction conductance as a function of time of heating, for different temperatures, diffusion coefficients and the heat of self‐diffusion can be evaluated in the normal manner. A similar relationship has been obtained for two spheres or hemispheres initially pressed together to give a known contact conductance before heating.

A Simple Interferometric Test for Conical Flow
View Description Hide DescriptionIn conical flows the velocity, pressure, and density do not vary on lines through the vertex of the cone. In this paper it is shown that for interferograms of general conical flows δ(y, z)/z=f(y/z), where y and z are any set of Cartesian coordinates with origin at the image of the vertex of the cone, and δ is the fringe shift at y, z. Thus, for strictly conical flow, a graph of δ(y, z)/z versus y/z should be a single curve. This suggests a test for approximate conicity that requires very little computation. This test is applied to interferograms obtained from a number of approximately axisymmetric flows at various Mach numbers about cone‐cylinders in free flight. Plotted fringe shift data from the region near the nose fall into a narrow band, an indication of approximate conicity. They also closely check the corresponding theoretical fringe shift curve calculated for Taylor‐Maccoll flow.

Theory of Collinear Antennas
View Description Hide DescriptionIf N collinear antennas are individually center driven, the only transmission line in the neutral plane is the one for a central unit if N is odd. Since lines that are not in a neutral plane constitute antennas, such an array is not practically useful.
For a single driven unit with symmetrical, parasitic, collinear elements, the driving transmission line is in the neutral plane and may be replaced by an equivalent generator and terminal‐zone network. An array with two parasites is analyzed. Since the currents in the halves of the three units are not alike, three simultaneous integral equations occur. By making reasonable assumptions these are reduced to the single equation of the isolated antenna but with a different kernel. Approximate expressions are obtained for the currents and for the mutual and self‐impedances of the elements.
The collinear antenna with outer elements driven from the central one by phase‐reversing stubs is studied. For three elements components of the currents in the several possible modes are obtained with different degrees of approximation. The in‐phase currents in the antennas are greatest, and are obtained with considerable accuracy. The currents in the outer units are not symmetrical, but with elements of length λ/2 the odd components of the currents are shown to be negligible.

Beam Oscillations in an F‐M Cyclotron
View Description Hide DescriptionThis paper considers the radial and vertical oscillations of a synchro‐cyclotron beam. It is found that various oscillations of substantially constant or increasing amplitude may occur in certain regions of the field, in which there are angular or other asymmetries, though only one possible oscillation (that at n≃⅕) can occur in a field that is cylindrically and vertically symmetric. The conditions for the various oscillations, and their behavior, are discussed.

On the Theory of Dislocations
View Description Hide DescriptionIf, in a multiply‐connected elastic solid, discontinuities are permitted across a stationary barrier in either the strain or its first derivatives or both, dislocations of a more general type than encountered in classical theory are possible. A number of these more general dislocations have been obtained for states of plane and anti‐plane strain in a hollow right circular cylinder when the surface of discontinuity is a single stationary plane barrier. Some of the dislocations found possess the characteristic that although the strain is continuous across the barrier the displacement discontinuity is not one which would be possible in a rigid body. Examination of the conditions for the uniqueness of solution of the boundary value problems of elasticity reveals that when dislocations of the more general type are admitted appropriate data must be given at each point on the specified barrier in addition to the usual information.

A Microwave Study of the High Pressure Arc
View Description Hide DescriptionThe impedance of an atmospheric pressure d.c. arc was measured at 1000 mc/sec. by means of a coaxial line and standing wave detector. The reactance and resistance of the arc, an essentially lumped load at the end of the line, increase with length of arc. The resistance decreases with increasing current and is approximately the same as the d.c. ``resistance.'' The reactance is capacitive and nearly independent of current. Air, argon, and helium were studied at currents from 1–4 amp. d.c. arc current.

A Relationship between Resistance and Temperature of Thermistors
View Description Hide DescriptionThe equation logR=A+B/(T+θ) for the resistance R of a thermistor at T°K is proposed. Least square analyses of the most precise resistance‐temperature data available for three different thermistormaterials show that the equation is a considerable improvement over its predecessors; the standard relative errors of fit are 0.4 percent, 0.17 percent, and 0.91 percent.

A High Intensity Short Duration Spark Light Source
View Description Hide DescriptionThe spark described here provides a high intensity light source, whose effective duration approaches 10^{−7} second. The light intensity is nine hundred times greater than that obtained using RG8U cable cut for 10^{−7} second duration discharge. The cable described here is coaxial with silver conductors fired on the dielectric, a compound of barium titanate. The gain in light intensity is apparently due to both the higher capacity of the barium titanate cable and its very low characteristic impedance.

Emissivity Changes of Thoria Cathodes
View Description Hide DescriptionSpectral and total emissivitymeasurements of thoria‐coated tungsten filaments show that irreversible and reversible changes of the emissivity occur. ελ (λ=0.65μ) varies from 0.2 to 0.7 and ε_{ t } from 0.2 to 0.5.
Certain variations of the thermal radiation can be related to changes of thermionic emission.
Passage of current through the coating affects emissivity and thermionic emission. This is shown by measuring infra‐red radiation and electron current simultaneously. The existing relationship between emissivity and thermionic activation is discussed.

Techniques for Measuring the Dynamic Characteristics of a Low Pressure Discharge
View Description Hide DescriptionThe dynamic characteristics of a discharge can be determined by making square‐wave changes of arc current. This is done by putting the discharge tube in the plate circuit of a group of pentodes connected in parallel and supplied with a square‐wave signal superimposed on a d.c. bias.
Probes operated at constant current are very useful in recording the characteristics of a modulated discharge. For measuring electron temperature (T_{e} ), two identical probes are centered about points having the same space potential. If the probes are operated at different fixed currents on the straight line part of the probe characteristic, the voltage between them is proportional to T_{e} . Longitudinal voltage gradient or radial potential differences may be recorded by use of identical probes operated at the same current.
A probe operated at a suitable current will follow roughly the variations in space potential of the adjacent region of the discharge. Another probe made sufficiently negative with respect to such a probe will collect only positive ions. A probe made 0–10 volts positive with respect to the constant‐current one gives records from which electron concentrations can be computed. In this case, a bias connection without current flow is obtained by use of a cathode‐follower with two stages of direct‐coupled amplification.
The oscillograph amplifier used for dynamic probe records must be either a differential one or one with ungrounded power supply. In the latter case, the stray capacitance is many times larger; but fairly reliable records can be obtained if the arc current is 0.1 ampere or more, and if the time constant for the decay of the transient effects being measured is at least 10 microseconds. The oscilloscopeamplifier and any preamplifier used for recording spectral intensities should be direct‐coupled.

On the Radiation Patterns of Dielectric Rods of Circular Cross Section— the TM _{0,1} Mode
View Description Hide DescriptionThe radiation patterns have been measured for five dielectric rods of circular cross section which were excited in the TM _{0,1} mode. These rods were each 0.870λ_{0} in diameter, and the lengths were 2λ_{0}, 4λ_{0}, 6λ_{0}, 8λ_{0}, and 10λ_{0}. The theoretical radiation patterns have been computed by means of equivalent surface electric and magnetic currents. Excellent agreement is obtained when it is assumed that the diameter of the surface on which these currents are distributed is 0.65 of the diameter of the dielectric rod. Representative experimental and theoretical patterns are shown for the full 360°.

Precision Determination of the Lattice Constants of Zinc Oxide
View Description Hide DescriptionA total of 350 back‐reflection ZnO powder x‐ray diffraction lines on 25 photographic films have been measured and analysed by the Cohen analytical least squares extrapolation method to yield a=(3.2495±0.0000_{2})A, c=(5.2069±0.0001_{1})A; where the stated errors are the probable errors. The statistical reliability of the results is emphasized.
In the measurement of the Debye‐Scherrer ring radii the accuracy obtained with a metric scale and indicator (employing no optical lens system) and with a dividing engine equipped with a microscope are compared and the former simpler method is found more reliable. It is concluded that the use of precision measuring instruments for observing diffraction ring radii are unnecessary and even undesirable apparently because the magnifying power of the optical system of precision micrometers leads to uncertainties in settings on photographic lines.

The Virtual Mass of a Sphere Entering Water Vertically
View Description Hide DescriptionThe manner in which a sphere behaves when it enters the water from air depends on the rate at which energy is taken from it, and most of this energy is utilized in setting water into motion. The virtual mass is a partial measure of this motion. An investigation of virtual mass for spheres shortly after vertical water entry is described. The spheres used had specific gravities between 1.06 and 16.77 and impact velocities ranged from 21 to 103 ft./sec. The value of the virtual‐mass coefficient was found to be much smaller than had previously been estimated, an average value of 0.08 being obtained. The method used recognizes the dependence of C_{D} on sphere speed and depth, and makes comparisons only where these parameters have the same value.

Plastic Flow of Platinum Wires
View Description Hide DescriptionThe strain‐time behavior of annealedplatinum wires at constant stress and temperature is reported for a wide range of experimental conditions, namely: 1550 to 78°K, 900 to 40,000 p.s.i., 0.001 to 0.1 strain, 10^{−1} to 10^{−6} min.^{−1} strain rate.
The following relations were found to describe the results satisfactorily:where σ=stress, ε=strain, σ=strain rate, T=absolute temperature, Q=activation energy for rate of deformation, t=time, and A, B, C, a, n, and p are constants.

The Effect of Magnetic Field on the Breakdown of Gases at Microwave Frequencies
View Description Hide DescriptionThe effect of magnetic field on the high frequency breakdown of gases has been studied. The presence of energy resonance and the modification of diffusion are shown experimentally and explained theoretically. An application is made of both the average electron theory and the Boltzmann theory, and the correspondence between these two theories is discussed.

Quantum Limits of the Electrostatic Image Force Theory
View Description Hide DescriptionThe quantum mechanical problem of the interaction of a charged macroscopic body with a metal surface is treated by means of the variation method. The resulting expression for the interaction energy is the same as that obtained by the classical image force method except for three correction terms. These corrections are caused by the change in kinetic energy associated with the concentration of the electrons on the surface of the metal, the reduction in the electron‐electron interaction energy in the metal resulting from the anti‐symmetrization of the wave function and the effect of the finite thickness of the surface charge. It is found that these terms are negligible except for distances between the charged body and metal surface such that surface structure effects are important.