Volume 17, Issue 8, 01 August 1946
Index of content:
17(1946); http://dx.doi.org/10.1063/1.1707764View Description Hide Description
A survey of the results obtained in an experimental study of the pulsed properties of oxide cathodes is presented. Pulsed measurements reveal that unusually large electron currents are available in microsecond pulses, and that several other phenomena are modifications of the d.c. properties, namely (a) sparking and (b) pulse temperature rise. Sparking may be either current limited or voltage limited, depending upon cathode materials and life. The pulse temperature rise also depends upon materials and life and is indicative of the nature of the cathode resistance. Evidence for a layer structure of the oxide cathode can be drawn from pulsed data.
17(1946); http://dx.doi.org/10.1063/1.1707765View Description Hide Description
X‐ray diffraction methods were used to investigate time changes which occur in the composition of oxide cathode coatings, initially equal molar (BaSr)O. Changes which occur in the composition of the bulk of the coating were detected by means of lattice constant measurements. A new method of analysis was developed to permit a determination of the variation of composition with depth below the surface. The bulk loss of BaO is primarily a function of the base metal used and the surface loss from the oxide is effected to a lesser extent. Possible correlations with thermionic emission are discussed.
17(1946); http://dx.doi.org/10.1063/1.1707766View Description Hide Description
X‐ray diffraction patterns are presented as evidence for the existence of a crystalline ``interface'' compound between the base metal and the coating of the oxide coatedcathode.Measurements of the coatingresistance to microsecond pulse currents, made with imbedded probes, indicate the presence of an anomalous ``interface'' resistance between the metal and coating. The relative magnitude of the coating and interface resistances are shown as a function of current for various operating temperatures.
17(1946); http://dx.doi.org/10.1063/1.1707767View Description Hide Description
17(1946); http://dx.doi.org/10.1063/1.1707768View Description Hide Description
A new method was developed for preparing natural and synthetic rubber specimens for examination with an electron microscope. The technique consisted of mixing latex with a water solution of polyvinyl alcohol and forming a film from the mixture. Electron micrographs of such specimens showed none of the flattening of particles characteristic of the collodion film method and the micrographs were suitable for direct particle‐size measurements.
Perturbation Theory of the Normal Modes for an Exponential M‐Curve in Non‐Standard Propagation of Microwaves17(1946); http://dx.doi.org/10.1063/1.1707769View Description Hide Description
In this paper a perturbation method is developed for treating non‐standard propagation of microwaves beyond the horizon in the case when the deviation of the M‐curve from the standard (≡ the M‐anomaly) can be represented by a term αe−λz , where z denotes height in natural units. Here M denotes the modified index of refraction of the air. The method is also applicable to other forms of the M‐anomaly which can be derived from an exponential term by differentiation with respect to λ; in fact, in its region of convergence it is formally applicable to the most general type of M‐curve, including elevated ducts. The region of practical convergence of the method ranges from highly substandard conditions down to cases where the decrement is a fraction of the standard value. The procedure followed is to express the height‐gain function Uk (z) of the kth mode in the non‐standard case as a linear combination of the height‐gain functions Um 0(z) of all the modes in the standard case:.The execution of this plan hinges on the possibility of evaluating the quantities.It is shown that β nm (λ) satisfies the differential equation,whose solution isHere Dm 0 denotes the characteristic value of the mth mode in the standard case. For large λ the following asymptotic formula holdsHaving determined the β nm (λ) from (d), or by a numerical solution of (c), the characteristic values Dk and the coefficients Akm are to be solved from the infinite system of equations,where δ nm denotes the Kronecker symbol. For this purpose a simple iterative procedure has been developed, which has been found to be rapidly convergent. The Akm are normalized by the condition.[The integral ∫0 ∞ Uk 2(z)dz diverges when taken along the real axis; it converges, however, and to the same limit, when the path is a radial line in the fourth quadrant of the z plane. In the sequel, whenever an integral is divergent, it will be understood that the path is suitably modified.] One can also expand Dk as a power series in αAn alternative expression for Dk (2) is given in Eq. (58).
17(1946); http://dx.doi.org/10.1063/1.1707770View Description Hide Description
It was shown previously that, when a cylinder of glass is sealed to the outside of a metal rod, the principal stresses in the glass are of opposite sign, so that tensile stresses cannot be avoided except by a perfect match. In this article the stresses are calculated for a solid glass cylinder sealed to the inside of a metal cylinder. It is shown that the stresses are all of the same sign, so that a moderate mismatch in thermal expansion, with the metal expansion the greater, is allowable and perhaps desirable. Large differences in expansion should be avoided, because of the shearing stresses at the ends.
17(1946); http://dx.doi.org/10.1063/1.1707771View Description Hide Description
The x‐ray line broadening method of determining particle size was compared with direct measurement on electron micrographs. By controlled heating of the carbonate, magnesium oxide particles were prepared from 50 to 1000A in diameter. Particle size calculated from x‐ray data taken on a Geiger counter spectrometer agreed to ±10 percent with the microscope measurements. Mechanical mixtures of two different sizes were examined by the x‐ray method, but the particle sizes could not be determined unless the two maxima of the distribution curve were completely resolved.
17(1946); http://dx.doi.org/10.1063/1.1707772View Description Hide Description
This paper presents measurements of the forces required to drive a punch into plates of several grades of steel as a function of the distance of penetration, the entire punching operation being conducted in a fluid medium subjected to hydrostatic pressures up to 30,000 kg/cm2. Qualitatively, the effects are similar to those already found for the tensile properties of steel subjected to hydrostatic pressure, namely, ductility is greatly increased, and greatly increased distortion is tolerated without fracture. The effects are similar in general character for all the steels experimented on here, but are quantitatively accentuated for the softer steels. At a pressure of 20,000 kg/cm2 or more a punch may be driven completely through a plate of mild steel, with no loss of coherence at any stage of the process, and with strain hardening, when expressed in terms of true shearing stress, which may increase by a factor of as much as 3. If the punching operation is suspended at any intermediate stage before complete penetration and afterward completed at atmospheric pressure, very material strengthening will be found as compared to virgin material of the same geometrical configuration. There are certain qualitative differences in the details of the ductility exhibited during the processes of punching and pulling. At intermediate pressures and penetrations the true shearing stress in punching may, under proper circumstances, exhibit maxima, for which there is no analog in the pulling operation. This is to be understood in terms of a difference of geometry, there being greater opportunity for self‐healing during the punching operation, and, furthermore, partial deterioration of the coherence of the metal not necessarily leading to complete catastrophe.
17(1946); http://dx.doi.org/10.1063/1.1707773View Description Hide Description
A method of evaluating mechanical hysteresis, elastic modulus, elongation, and growth rate of a yarn or cord subjected to cyclic loads in a fatigue test is described. The test is applied to seven typical tire cords‐one Nylon, two rayon, and four cotton cords including both high stretch and low stretch cords. Results are presented showing the effect of cycle number, moisture content, temperature, and load range on the elastic properties of each of the cords tested. The influence of cyclic loading on the stress‐strain curve is also discussed. It is concluded that moisture content is the dominant factor influencing the elastic properties of tire cords and that the heat produced by mechanical hysteresis of the tire cord contributes appreciably to the heat build‐up observed in tires during heavy duty service.