Index of content:
Volume 25, Issue 4, 01 April 1954
Electromagnetic Velometry. II. Elimination of the Effects of Induced Currents in Explorations of the Velocity Distribution in Axially Symmetrical Flow25(1954); http://dx.doi.org/10.1063/1.1721654View Description Hide Description
The distribution of electrical potential in a conduit carrying an axially symmetrical flow through an homogeneous magnetic field, which is perpendicular to the flow, is analyzed. It is shown that it is possible to eliminate the difficulties arising from induced currents in electromagnetic determinations of the velocity distribution by measuring the potential gradient ∂ψ/∂y (y being ⊥H and ⊥v) along a diameter bisecting the angle between the H axis and the Y axis. The knowledge of the distribution of (∂ψ/∂y)45° at a known discharge through the conduit determines the velocity distribution v=f(r).
25(1954); http://dx.doi.org/10.1063/1.1721655View Description Hide Description
By means of the vector calculus, it is proved that the magnitude, orientation, and location of the resultant dipole of a system of sources and sinks inside a finite volume conductor is given by an integration over the bounding surface. The method is applied to finding the ``heart vector,'' or the resultant dipole moment of the human heart. The theory was checked in two‐ and three‐dimensional electrolytic tank models of the human thorax.
25(1954); http://dx.doi.org/10.1063/1.1721656View Description Hide Description
Breakdown field strengths have been measured in air and hydrogen at pressures of the order of 1 micron Hg and frequencies from 25 to 90 Mc/sec between flat metal electrodes at separations from 1 to 4 cm. By suddenly applying a high voltage and then lowering it slowly an upper breakdown curve has been observed. This new curve can be combined with the lower breakdown curve and cut‐off frequency such as reported previously by the Guttons, and by Gill and von Engel to form the boundaries of a breakdown region in the frequency‐field strength domain.
The simple secondary electron resonance theories developed by Danielsson and Gill and von Engel have been extended by the semi‐empirical determination of minimum and maximum limiting values of the electron‐emission phase angle, a minimum limiting value of electron arrival energy, and a ratio between electron arrival and emission velocities.
25(1954); http://dx.doi.org/10.1063/1.1721657View Description Hide Description
Switching or triggering in certain nonlinear circuits having two stable states is studied in graphic detail. The minimum and maximum pulse requirements are discussed and some of the general ideas of bistability considered. A convenient technique for studying the response of circuits, apparently in slow motion, is described.
25(1954); http://dx.doi.org/10.1063/1.1721658View Description Hide Description
The qualitative explanation by Loeb of the constancy of the amplification factor with increasing corona currents with coaxial cylindrical electrodes used, as in conventional Geiger counters, is placed on a quantitative basis. As anticipated, it is shown that it is the screening action of the space charge which is responsible for the constancy and the stability of the average discharge current. The fluctuations superposed on the constant current, previously reported by us, are explained as a response of the system having a definite resonance frequency to the statistical fluctuations of the photoelectric current. The theoretical predictions are confirmed by experimental data.
25(1954); http://dx.doi.org/10.1063/1.1721659View Description Hide Description
The problem of focusing long electron beams by means of magnetic or electric fields which vary periodically along the beam is considered. Four specific cases are discussed: periodic fields of the axially symmetric and quadrupole type with either electric or magnetic fields employed. The equations of motion are written and solutions obtained which show beam trajectories corresponding to essentially parallel flow. Under certain specified conditions only small ripples are present. Actual trajectories as plotted by the analog computer are shown and charts and equations are presented to aid in the design of practical periodic focusing systems.
25(1954); http://dx.doi.org/10.1063/1.1721660View Description Hide Description
A method is described for constructing germanium high‐voltage diodes, utilizing 15‐mil germanium spheres instead of the usual flat wafer. The spheres are produced within a few minutes in quantities of several hundred thousand or more by blowing molten high‐purity germanium from a graphite crucible. The spherical pellets can be annealed, ground, etched, and assembled into diodes by techniques easily adaptable to automatic mass production. The assembled diode uses a sphere‐plane contact rather than the conventional whisker contact. Peak back‐voltages in the range 50–100 volts are easily obtainable. Series‐assembled units for higher voltages are easily made.
25(1954); http://dx.doi.org/10.1063/1.1721661View Description Hide Description
Part I is a description of some of the factors involved in the recovery by pure zincsingle crystals at 35°C of the ability to creep following plastic deformation. The following observations were made: (1) Complete recovery of the ability to show an initial instantaneous plastic extension occurs much less readily than recovery of the ability to show transient creep. (2) Once the quasi‐viscous creep stage is reached no recovery of transient creep occurs when the load is partially or completely removed for two minutes, but removal of only 6 percent of the load for 100 minutes permits partial recovery of transient creep at the full load. A 300 minute ``rest'' at 91.5 percent of the full load resulted in complete recovery of the transient creep at full load, even though creep occurred at the reduced stress. (3) The recovery of quasi‐viscous creep at a reduced stress after exposure to a higher stress was also investigated. The recovery was slower, the larger the initial load. (4) The rate of the time‐dependent contraction of zinc crystals on removal of the load probably follows a power law in the time. There is also an instantaneous nonelastic contraction which is about ten times larger than the elastic contraction.
These results show that the role of thermal fluctuations in activating the units of flow is very different from the role of the stress. The applications of the Eyring rate equation to quasi‐viscous creep which have been made in the past are, therefore, not valid. A variable activation energy must be postulated in order to account for the kinetics of creep and recovery. These results are interpreted in terms of the dislocation theory of plastic flow.
In Part II a phenomenon is described in which pure zincsingle crystals are consistently found to become temporarily hardened by annealing in vacuum above 200–260°C. The hardness thus introduced can be removed by straining the crystal and then allowing it to stand at temperatures below 200°C. The crystal is in this way returned to its normal plastic state. The hardening is not removed by electropolishing the crystal, so is not a surface phenomenon. Though similar to Orowan's thermal hardening effect, it differs by requiring a much higher temperature and by not requiring the addition of impurities.
25(1954); http://dx.doi.org/10.1063/1.1721662View Description Hide Description
Germanium metal, both single crystalline and polycrystalline, is frequently found to contain sharp fluctuations in impurity concentration. These are superimposed on the gradual variations expected as the result of ordinary segregation processes.
A rather simple technique for detecting fluctuations of this kind has been developed. It is more sensitive than resistivity scanning and does not involve the use of radioactive tracers. This technique has been applied to a number of samples. The origin, value, and elimination of these fluctuations are discussed briefly.
25(1954); http://dx.doi.org/10.1063/1.1721663View Description Hide Description
The resistance experienced by a curved and elongated small particle is studied by the method of velocity perturbations. The fundamental equations determining the perturbations are derived from the equations of Oseen. The particle is curved in such a way that the axis of the particle is formed by an arc of circle. The opening angle may vary to exhibit various shapes from a straight ellipsoid to a closed ring. Results illustrate the circumstances under which manifests the cooperation between the different parts of the particle. Apart from this pure hydrodynamical interest, the problem has applications to long chain molecules in solution and to suspensions of swimming organisms. It demonstrates the effect of skew shape by taking for the chain molecule a model in the form of a variable‐curved ellipsoid instead of a sphere or a cluster of spherical symmetry, as the coordination of the chain links, even in the case of free rotation, leads initially to configurations of variable skewness generally obscured under the averaging processes by current theories.
25(1954); http://dx.doi.org/10.1063/1.1721664View Description Hide Description
The integro‐differential equation for the growth of a vapor bubble in a superheated liquid is formulated and discussed. It is shown that two distinct time domains exist: one, of the order of 10−4 second, during which the effect of the hydrodynamic forces may be an important factor in the growth of the bubble, and another, during which this effect is unimportant. An integral equation is formulated for the latter domain. A solution of the problem, in closed form, valid for the entire interval of interest is presented; it agrees very well with experimental data for various superheats.
25(1954); http://dx.doi.org/10.1063/1.1721665View Description Hide Description
Rectangular hysteresis‐loop magnetic materials are widely used at present for the storage of digital information. The principles involved in static magnetic storage are generalized in this paper so that magnetic cores may be used for the purpose of switching. Magnetic circuits to produce the logical operations of ``and'' and ``or'' are described. The ``and'' and ``or'' circuits are combined into a class of circuits capable of producing any switching function of any number of input binary variables. All of the magnetic switching circuits described are devoid of rectifiers, vacuum tubes,transistors, and the like. Furthermore, these circuits are essentially independent of pulse amplitudes and durations.
A Graphical Method of Evaluating Trigonometric Functions Used in Crystal Structure Analysis. Parts I and II25(1954); http://dx.doi.org/10.1063/1.1721666View Description Hide Description
A graphical method is described for evaluating structure factors of the form and electron density expressions of the form . The method depends upon the fact that if, in a  projection of a unit cell the normal to the planes (hk0) is graduated with the appropriate sine or cosine function giving the phases of points along the normal, then a circle with its center at (½x,½y), passing through the origin, will cut the normal to F(hk0) at the value of . Part I contains the basic principles and a standard chart, and Part II cites examples of the use of the method and the accuracy to be expected in structure factor calculations, and of its application to least‐squares refinements.
25(1954); http://dx.doi.org/10.1063/1.1721667View Description Hide Description
25(1954); http://dx.doi.org/10.1063/1.1721668View Description Hide Description
The growth of a vapor bubble in a superheated liquid is controlled by three factors: the inertia of the liquid, the surface tension, and the vapor pressure. As the bubble grows, evaporation takes place at the bubble boundary, and the temperature and vapor pressure in the bubble are thereby decreased. The heat inflow requirement of evaporation, however, depends on the rate of bubble growth, so that the dynamic problem is linked with a heat diffusion problem. Since the heat diffusion problem has been solved, a quantitative formulation of the dynamic problem can be given. A solution for the radius of the vapor bubble as a function of time is obtained which is valid for sufficiently large radius. This asymptotic solution covers the range of physical interest since the radius at which it becomes valid is near the lower limit of experimental observation. It shows the strong effect of heat diffusion on the rate of bubble growth. Comparison of the predicted radius‐time behavior is made with experimental observations in superheated water, and very good agreement is found.
25(1954); http://dx.doi.org/10.1063/1.1721669View Description Hide Description
The charging of borosilicate glass spheres rolling on clean nickel has been studied under controlled conditions of cleanliness, humidity, and gas pressure. Strong evidence indicates the process of charge transfer to be a type of contact electrification akin to that operating between two metals. The observed charging is, however, influenced by other factors. The amount of charge acquired increases with surface areas in contact and hence with distance rolled until other factors intervene. The rate of charging depends on rolling speed and surface conductivity of the borosilicate glass, while the maximum equilibrium charge depends on the pressure of the ambient gas. The gas pressureeffect is shown to be caused by electrical discharge of the highly charged sections of the borosilicate glass to the metal, as suggested by E. W. B. Gill, and this causes a minimum equilibrium charge to occur about 1 mm air pressure, as observed by D. E. Debeau. The effect of transverse electrical fields reported by E. W. B. Gill and G. F. Alfrey is negligible for low surface conductivity but becomes important at higher surface conductivity, in particular above 40 percent relative humidity at 20°C.
25(1954); http://dx.doi.org/10.1063/1.1721670View Description Hide Description
A simple method of estimating differential angular scattering cross sections in an infinite medium is presented. It assumes that single scattering theory approximates the radiation flux at a detector which is separated from the source by much less than a mean free path.
25(1954); http://dx.doi.org/10.1063/1.1721671View Description Hide Description
The radiationbackscattered by semi‐infinite slabs of wood and steel wool, when irradiated by the Co60gamma rays, has been investigated by means of a sodium iodide scintillation spectrometer. The main contributions to the scatteredradiation are produced in one or two Compton collisions. However, the radiation that has suffered so many collisions as to have an isotropic angular distribution has also been observed.
25(1954); http://dx.doi.org/10.1063/1.1721672View Description Hide Description
The work of O'Rourke and Saenz on the photoelastic determination of quenching stresses is re‐examined and extended. A brief general discussion is given on the determination of shear stresses or stress differences in the interior of a photoelastic model from retardation patterns obtained by passing light through the entire model. A method is proposed for the cylindrical case, with no variation in stress along the axis, which is feasible at present, while the proposal for the general axi‐symmetric problem is feasible in principle. The fundamental difficulty of the general three‐dimensional model is brought out but no solution is advanced.
25(1954); http://dx.doi.org/10.1063/1.1721673View Description Hide Description
A transistor structure utilizing a planar p‐n junction as a collector, and a point contact as an emitter has been analyzed theoretically and experimentally. The theory indicates that for maximum frequency cutoff the plane containing the emitter point should be nearly parallel to the collector junction. Also it is found that the base resistance is a critical function of the spacing between the emitter point and the collector junction. Results for experimental models are discussed.