Index of content:
Volume 27, Issue 11, 01 November 1956
27(1956); http://dx.doi.org/10.1063/1.1722248View Description Hide Description
In conventional coincident‐current magnetic storage systems, a specific storage element is selected by simultaneously applying, to two or more intersecting wires, currents of magnitudes such that their sum will produce a magnetic field sufficient to switch the element and such that the presence of any one of the selecting currents alone will not cause the element to switch. In the system described here, a coincident summation of magnetic flux causes the element to switch, and the magnitudes of the currents inducing the flux are not critical. This allows the switching time to be freed of the current‐selection‐ratio limitation typical of the coincident‐current selection systems. In the coincident‐flux system, one may use selecting flux densities representing fields very much greater than the coercive field, giving very fast switching times.
27(1956); http://dx.doi.org/10.1063/1.1722249View Description Hide Description
The role of crack propagation velocity in the cleavage of crystals and aggregates is emphasized in this paper. The available data on crack propagation in elastic media are summarized, showing that this phenomenon is adequately understood at present. For inelastic media, it is postulated that in addition to the Griffith Criterion two critical velocity conditions must be satisfied or crack propagation cannot occur. One critical velocity pertains to propagation through a crystal, and the other to propagation through an aggregate. The first critical velocity depends on the rate at which plastic glide can absorb energy at a crack tip and the number and size of cleavage steps that a crack front contains. These quantities are estimated. The second critical velocity accounts for the dependence of fracture stress on grain size in aggregates and interprets other experimental behaviors of aggregates which contradict previous interpretations.
27(1956); http://dx.doi.org/10.1063/1.1722250View Description Hide Description
Specimens of U‐9, ‐10.5, ‐12, and ‐13.5 w/o (weight percent) Mo and U‐10 w/o Nb, heat treated to retain the metastable gamma phase at room temperature and also to transform this phase to the room temperature stable phases, were irradiated in the Materials Testing Reactor (MTR) at maximum temperatures of less than 200°C. Electrical resistivity, temperature coefficient of electrical resistivity,hardness, and density measurements as well as x‐ray diffraction patterns indicated that the stable phases at room temperature reverted to the metastable gamma phase during irradiation. No apparent changes in the microstructures of thermally transformed specimens were observed after irradiation. Lack of observable microstructural changes was attributed to the development of a ghost structure. X‐ray line broadening measurements indicated essentially complete homogenization of thermally transformed U‐Mo alloy specimens during irradiation. The mechanism of the phase reversal was explained on the basis of the displacement spike hypothesis.
27(1956); http://dx.doi.org/10.1063/1.1722251View Description Hide Description
A mathematical treatment of the Bennett‐type rf mass spectrometer has been developed which is appreciably simpler to handle when dealing with a multistage analyzer. This simplification is a result of changing the point of reference in the tube from that previously used. The resulting form of the equation is soluble by the use of a very simple analog computer which is described.
27(1956); http://dx.doi.org/10.1063/1.1722252View Description Hide Description
Effects of crystal growth rate and growth direction on the density of random dislocations and on the minority carrier lifetime have been observed. The dislocation density increases rapidly with growth rate above a rate of about 0.15 in. per minute and varies somewhat with growth direction. The capture efficiency per unit length of dislocation decreases at high growth rates and it is suggested that this effect is caused by the failure of impurity atoms to segregate at dislocations or by the clustering of dislocations.
27(1956); http://dx.doi.org/10.1063/1.1722253View Description Hide Description
The variance of the number of zeros of a Gaussian signal on a short time interval was derived in a recent paper by Steinberg, et al. This result is generalized to include the covariance of the number of zeros of a Gaussian signal at the values θ1 and θ2, using a somewhat different mathematical approach. A special case with θ1=θ2=θ gives the variance of the number of times the signal equals θ on a given time interval.
27(1956); http://dx.doi.org/10.1063/1.1722254View Description Hide Description
A general method is developed to allow calculation of the critical buckling of an assemblage of fissile regions coupled by neutron leakage. Consideration is given both to regions embedded in vacuum and in material media. Several cases of simple geometry are further developed to illustrate the theory.
27(1956); http://dx.doi.org/10.1063/1.1722255View Description Hide Description
An investigation has been made of the electrostatic charge produced in vacuum on several inorganic insulators when they are put into rolling contact with nickel,platinum, and coppersurfaces. The primary role of relative motion is to increase the area on the insulator that ultimately undergoes contact, thereby increasing the total charge separated. With quartz, all evidence favors electron transfer which occurs because the nonconductor has a higher effective work function than the metal. Field emission‐induced back leakage of electrons during separation of the charged surfaces is considered to reduce the charge originally separated to that actually measured. Little difference is seen between the charging properties of quartz cleaned chemically in room air and quartz subsequently outgassed at 700°C in vacuo better than 10−8 mm Hg. No difference in electron affinity is found between fused and monocrystalline quartz. No evidence directly supporting electron transfer is found for the other insulators tested, synthetic single crystals of Al2O3, MgO, NaCl, KCl, KBr, and KI; and the possibility of ion transfer is considered. All charge against nickel much less vigorously and less reproducibly than does quartz. The alkali halides and MgO which contains excess Mg or O as a bulk impurity charge positively, while stoichiometric MgO charges negatively. With Al2O3, there is a strong dependence of charge density on crystallographic orientation. Alteration of charging properties by surface and/or bulk conduction is found to be negligible for all insulators tested, within the reproducibility of the charging measurements.
27(1956); http://dx.doi.org/10.1063/1.1722256View Description Hide Description
Some simple methods of preparing MgO thin films on Mg–Ag alloy with high secondary emission ratios are discussed. The oxidation procedure and a discussion of the underlying phenomena are presented. Films with a secondary yield of 12 at 600 v may be produced by oxidation in either oxygen or carbon dioxide. Results of a study of some of the factors influencing the life of a secondary emitter are given. These include the effects on the secondary emitting characteristics of evaporation products from an oxide‐coated cathode and the deterioration of the thin film under electron bombardment.
27(1956); http://dx.doi.org/10.1063/1.1722257View Description Hide Description
To reduce the physical and chemical complexity of the oxide coatedcathode, the support metal and interface were eliminated by applying the (BaSr)O layer to a MgO ceramic which is electrically and chemically inert to (BaSr)O. The three interacting chemical phases of the usual oxide coatedcathode are thereby reduced to a single phase—the oxide coat alone. At 1000°K, thermionic emission, measured by a pulse technique, is about 1 order of magnitude less than from modern commercial cathodes but considerably greater than from any other cathode system. The emission capability is unaffected by treatment in either atomic hydrogen or methane, which reduce BaO to Ba metal and thereby increase the Ba content of the oxide. The experiments were an attempt to test the usual assumption that emission in conventional oxide coatedcathodes is determined by the composition of the (BaSr)O phase. The results suggest strongly that much of the complexity of practical cathodes is in the oxide layer itself and that factors other than the content of excess Ba are decisive for thermionic emission.
27(1956); http://dx.doi.org/10.1063/1.1722258View Description Hide Description
The progress of an isothermal chemical reaction on a catalytic surface, which is located in a laminar hydrodynamic flow field of large Reynolds number, is analyzed. Past analytical investigations have been restricted primarily to chemical reactions in fully velocity developed flow fields subject frequently to one or both of the following approximations. (i) The actual velocity distribution is replaced by a flow of uniform velocity. (ii) The concentrations of the reactants on the catalytic surface are prescribed subject to certain hypotheses concerning the rate controlling mechanism.
In this investigation the principal aim has been to calculate the actual surface concentration in a laminar boundary layer flow without unduly sacrificing the hydrodynamic features or introducing unnecessary assumptions about the effective over‐all reaction mechanism. The method presented, which is related to an analysis of convective heat transfer from a nonisothermal surface, is applicable to arbitrary catalytic reaction mechanisms in a certain class of laminar boundary layer type flow fields.
27(1956); http://dx.doi.org/10.1063/1.1722259View Description Hide Description
A thin circular ferrite post magnetized lengthwise is placed in a rectangular wave guide with its axis normal to the direction of propagation of the incident waves. The polarization is such that the electric vector is parallel to the post. The reflected and transmitted waves are calculated both with respect to their intensities and phases. The results are also applied to find the influence of a thin ferrite post upon the resonant frequency of a rectangular cavity.
Quantitative Study of Substructure Characteristics and Correlation to Tensile‐Property of Nickel and Nickel Alloy27(1956); http://dx.doi.org/10.1063/1.1722260View Description Hide Description
The application of a method combining x‐ray microscopy and diffraction analysis to the study of the substructure of nickel and nickel alloys disclosed the coexistence of three orders of magnitude of substructural entities. These three orders are structurally interrelated and distinguished by different ranges of size, disorientation angle, and lattice misalignment. The statistical analysis of the data disclosed that for the two largest orders of substructural entities increasing amounts of room temperature deformation cause (1) a decrease in the mean size and a decrease in the spread of the size distribution, (2) an increase in the mean disorientation angle between substructural entities of the same order and an increase in the spread of the distribution of angles, and (3) an increase in the mean lattice misalignment and spread in the distribution of misalignment. Progressive room temperature deformation causes the low‐angle boundaries to become increasingly unsymmetrical and greatly increases the number of the smallest substructural entities. The rise of flow stress as a function of room temperature deformation was correlated to the systematic modification of substructure characteristics.
27(1956); http://dx.doi.org/10.1063/1.1722261View Description Hide Description
The numerical solution for a spherical shock in water produced by spherical piston of known expanding motion and initial nonzero radius is described. This solution is of interest during the time when the radius of the shock and that of the piston are of the same order of magnitude, that is, when point source solutions do not hold. Calculations are carried out for a constant velocity piston and for a piston with decreasing velocity.
27(1956); http://dx.doi.org/10.1063/1.1722262View Description Hide Description
A modified Landau‐Lifshitz equation is solved for a single‐domain sphere and an infinitely‐wide thin single‐domain sheet of ferromagnetic material neglecting anisotropy. The external magnetic field is switched from one direction to its opposite instantaneously at the initial time and the behavior of the magnetization vector is investigated thereafter. It is shown that there is a critical value of the damping constant corresponding to the minimum value of the (repetitive) magnetization reversal time. If the damping constant is larger than the critical value, the magnetization vector moves slower; if it is smaller, the magnetization vector moves faster but oscillates so that it takes longer time until it comes to a rest at the final position. The critical values of the Landau‐Lifshitz damping constant λ are γM for the sphere and 0.013γM for the thin sheet, where γ and M are the gyromagnetic ratio and the magnetization, respectively. The computed minimum switching time for the thin sheet of 4–79 molybdenumPermalloy is of the order of 10−9 sec.
27(1956); http://dx.doi.org/10.1063/1.1722263View Description Hide Description
The resistance of the semitransparent photoemissive films Sb–Cs, Sb–Rb, Bi–Cs, Bi–Rb, Te–Cs, Te–Rb, and Ag–O–Cs was measured as a function of temperature. A thermal activation energy associated with conductivity was determined for each of the materials.
27(1956); http://dx.doi.org/10.1063/1.1722264View Description Hide Description
A new mathematical approach for the analysis of radioactive tracer experiments on compartmentalized systems in steady states is presented. The experimental measurements of the amounts of tracer in one or more compartments are approximated by a sum of exponential functions of time. The coefficients and exponential factors of these functions are shown to represent a set of invariants of the data. These are then related in a concise matrix equation to the compartmental model parameters, which are defined as the transition probabilities per unit time per unit quantity (turn‐over rates) of molecules from one compartment (or state) into another.
When measurements are incomplete, the freedom in choosing a model is expressed in turns of a minimum number of variables equal to the difference between the number of model parameters and the number of invariants found in the data. It is shown how other information regarding the model parameters or the amounts of material in steady state may be combined with the tracer data to reduce the degrees of freedom of a proposed model.
Matrix transformations are worked out to allow the mapping of all mathematically consistent models in the configuration space of the minimum variables. The boundaries in this space corresponding to the limits for physically realizable models are found. Thus, a model is expressed by a set of coordinates and the values of all the parameters are obtained by substituting the values of the coordinates. Investigation of the range of variation of each parameter over the bounded region is also possible.
27(1956); http://dx.doi.org/10.1063/1.1722265View Description Hide Description
By observation of rolling friction effects, the rate of drying of paints, varnishes, and other surface coatings can be evaluated more precisely than by previous methods. The time of descent of a ball rolling down a coated incline has been proposed as an index of surface condition. The basic physical phenomenon involved, however, is rolling friction, so the coefficient of rolling friction would be preferable for use as a standard. A metric analogous to Brinell hardness can be defined from the coefficient of rolling friction. The ratio between the coefficient of rolling friction and the radius of the test ball is proposed as an index of surface condition.
27(1956); http://dx.doi.org/10.1063/1.1722266View Description Hide Description
The generation of an even‐order subharmonic, or harmonic, oscillation in a driven nonlinear resonant system described by just a cubic nonlinear term requires the simultaneous appearance of a zero‐frequency component. This component effectively introduces a squared nonlinear term. A simple experiment with an electrical circuit is described, showing the presence of the zero‐frequency component.
27(1956); http://dx.doi.org/10.1063/1.1722267View Description Hide Description
Various kinds of silica glass and natural and synthetic crystals have been irradiated with fast neutrons at temperatures of ∼250°C and above 300°C. In the glasses, asymmetric paramagnetic resonance lines resulting from the irradiation have been observed with apparent g values of 2.0013±0.0006 and 2.0090±0.0007, and half‐widths of ≈1.7 gauss and ≈40 gauss, respectively. Two groups of lines were found in the natural crystals and have been associated with the two lines in the glasses, the two lines in the glasses being the envelope of the lines in the crystals when they are summed over random orientations with respect to the magnetic field. On the basis of g values, absence of common impurity and of hyperfine splitting, and the thermal stability of the lines, it is concluded that the observed resonances in the silica glasses and crystals have their origin in defects in the basic SiO4 tetrahedra generated by fast neutrons and primary and secondary knockons. From preliminary optical bleaching data in neutronirradiated Corning silica it is tentatively concluded that the resonance with g=2.0013 is associated with the defect which gives rise to an optical absorption band at 2120 A. The g values of the two lines in the silica glasses suggest that one center is an electron and the other a hole.