Volume 27, Issue 10, 01 October 1956
Index of content:
27(1956); http://dx.doi.org/10.1063/1.1722212View Description Hide Description
A new method is given for measuring dynamic plastic strain in metals under impact loading. Strain‐time curves for initial and reflected wave fronts have been determined using a gauge length of in. The measurements are made by observing the behavior during strain of the diffracted and central images of an 8300 line reflection grating ruled on the specimen surface.
27(1956); http://dx.doi.org/10.1063/1.1722213View Description Hide Description
Change of voltage across a solid insulator produces a transient electric current.Measurements of such currents from the polymers of methyl methacrylate, monochlorotrifluoroethylene, ethylene, styrene, and tetrafluoroethylene were made for times from 10 to 104 seconds after change in voltage at 25°C, 47°C, and several points below room temperature. The current produced by applying a constant voltage to an undisturbed specimen decays as the negative nth power of the time, where n is a constant between 0.7 and 1.1. The currents from polyethylene and polytetrafluoroethylene do not conform to the superposition principle, in contradistinction to what is usually reported for solid insulators. The currents at 100 seconds exhibit maxima at −32°C for polymethyl methacrylate and at −50°C for polymonochlorotrifluoroethylene. It is suggested that permanent electric dipoles play an important role in the currents from polymethyl methacrylate and polymonochlorotrifluoroethylene.
27(1956); http://dx.doi.org/10.1063/1.1722214View Description Hide Description
Theorems relating the plane wave modes (scalar or vector) transmitted and reflected by periodic structures of lossless elements bounded by two parallel planes are derived. One special case is continuity of energy flux normal to the boundary planes, and others relate the phases, or both phases and magnitudes, of the propagating modes. These relations are essentially discrete analogs (i.e., with integrals over all angles of observation replaced by sums over propagating mode directions) of the relations for a bounded scatterer given by Saxon. The essential difference in the two situations is that by restricting the parameters so that only a few modes propagate we obtain special cases which can be treated relatively explicitly. Thus the case of one propagating mode, and the cases corresponding to Bragg reflections are discussed in some detail; here there is both conservation of magnitude and phase.
27(1956); http://dx.doi.org/10.1063/1.1722215View Description Hide Description
A study of high speed, metal‐to‐metal impact in the velocity range 1 to 5 mm/μsec using ⅛‐inch diameter spherical pellets is described. Pellet materials include aluminum,magnesium, steel, brass,lead, and zinc. Experiments relating to the mechanisms of cratering and the perforation of thin targets are presented. For the ⅛‐in. diameter spherical pellets, it is found that the volume of the crater per unit energy of the impacting pellet is essentially constant for each material and that the penetration is proportional to the velocity of the pellet so long as the pellet velocity is less than the velocity of sound in the target material. An attempt to model very high speed impact by using soft wax targets in which the sonic velocity is less than the impactingvelocity is outlined.
27(1956); http://dx.doi.org/10.1063/1.1722216View Description Hide Description
The breakup of large water drops during free fall is of importance to meteorologists.Theories of precipitation lean heavily on drop multiplication resulting from the shattering of large drops. An experiment is described in which the actual breakup of large drops is observed, and data obtained from which the mechanism of break may be inferred. Drops in the various stages of disintegration have been photographed and the size distribution of fragments noted. A large drop falling freely through the air deforms, inflates somewhat in the same manner as a parachute and bursts with considerable violence. The origin of various size groups of fragments and its significance in determining the observed size distribution has been noted. An experimental arrangement is described that permits a large number of photographs to be taken of all stages of disintegration.
27(1956); http://dx.doi.org/10.1063/1.1722217View Description Hide Description
A stratified dielectric sheet with dielectric constant ε(x), permeability μ(x) and thickness d is uniformly contracted to yield a sheet with parameters ε(nx), μ(nx), d/n. Now n of these contracted panels are stacked up to yield again a sheet of thickness d. What can be said of the limiting medium obtained in this way, as n→∞ ? If the symbol  denotes ``mean value of,'' the results of this paper may be summarized as follows: At perpendicular polarization, the limiting medium acts like a homogeneous medium with dielectric constant, permeability and thickness given respectively by . At parallel polarization the parameters are, respectively, . The relevance of these results to topics of current interest is indicated. The analysis is not based on the equivalent‐circuit analogues which are sometimes used to study artificial dielectrics, but is developed without approximation from Maxwell's equations.
27(1956); http://dx.doi.org/10.1063/1.1722218View Description Hide Description
The free‐flight motion of a satellite around a spherical earth is investigated neglecting atmospheric deceleration. This treatment is then extended by recognizing the earth's oblateness, which is represented by an additional quadrupole field. For nearly circular orbits, one finds that perturbations of the usual planar motion may be described analytically. These are characterized by a precession of the orbit plane around the equator, that is, a regression of the nodes, amounting to as much as forty miles per revolution. The large magnitude of these effects indicates that one may exploit a satellite's motion, as measured by conventional radio techniques, to determine the earth's oblateness to a new precision.
27(1956); http://dx.doi.org/10.1063/1.1722220View Description Hide Description
When water flows over glass which has been treated with the vapor of dimethyldichlorosilane and thus made water repellent, slipping on the boundary between the solid surface and the water takes place. This is is shown in capillary tubes of various diameters. The amount of slipping is small, but measurable. It disappears or becomes extremely small in case of turbulent flow.
27(1956); http://dx.doi.org/10.1063/1.1722221View Description Hide Description
When an explosive detonates underwater it creates a bubble of gas which performs damped radial oscillations of large amplitude. The usual theory of these oscillations treats the water as incompressible and yields undamped oscillations of constant period. We have modified this theory by taking account of the compressibility of the water. Our theory predicts damped oscillations of diminishing period. Comparison of the predicted and observed radius‐time curves for one particular case shows fairly good agreement. Radius‐time curves for four representative cases have been computed with a large number of periods in each case. These can be used to describe a variety of explosions.
27(1956); http://dx.doi.org/10.1063/1.1722222View Description Hide Description
A theory for uniform field breakdown in air at microwave frequencies is developed and applied with success to predicting values of breakdown over a wide range of experimental conditions. Three distinct types of breakdown are treated; c.w. (continuous wave) breakdown, single pulse breakdown, and multipulse breakdown. The conditions for breakdown are determined from a solution of the electron continuity equation for an average electron, in which electron ionization, attachment, and diffusion are the dominant mechanisms. Modulation of the electron average energy at twice the frequency of the applied field becomes important at either high pressure or low frequency and modifies the values of the breakdown field. The breakdown field strengths are shown to be determined from a single curve for each type of breakdown power, either c.w. or pulsed. These theoretical curves are in accordance with the experimental results, thus verifying the assumption and the values of the coefficients used in the theory.
27(1956); http://dx.doi.org/10.1063/1.1722223View Description Hide Description
Experimental texture data were collected for a titanium and a uranium rod in order to test the applicability of the sinφ correction factor to be used with pole charts for fiber textures. Comparison of the corrected and uncorrected integrated intensity values for pole charts corresponding to various reflections with the calculated intensity values indicates that the agreement is greatly improved when the sinφ correction factor is used.
27(1956); http://dx.doi.org/10.1063/1.1722224View Description Hide Description
In contrast to graphite, MoS2 retains its lubrication properties in vacuum. The coefficient of friction of MoS2powder, compressed into pellets, has been measured under high vacuum conditions. Well run‐in surfaces, which have been stationary for a period of time, show higher friction initially upon resumption of sliding. Presence of an amorphous layer of sulfur during sliding is postulated.
27(1956); http://dx.doi.org/10.1063/1.1722225View Description Hide Description
Evidence is presented which seems to indicate that a major part of the flicker noise in tubes with oxide‐coated cathodes is generated in a thin surface layer of the coating. The effect is shown to be caused by the fact that a dc voltage drop and a noise voltage fluctuation are generated in the surface layer. In tubes with a porous cathode coating this noise voltage modulates the current coming out of the surface pores, thus leading to a true fluctuation in the current. In tubes with a nonporous cathode coating this noise voltage modulates the emission current, thus leading to a true fluctuation in the emission.
27(1956); http://dx.doi.org/10.1063/1.1722226View Description Hide Description
The dynamic elastic constants of Armco iron, pure aluminum, and fused quartz have been measured with a supersonic pulse technique as functions of pressure and temperature over the range 1 to 9000 bars and 25°C to 300°C (200°C for quartz).
Within the experimental accuracy the dynamic elastic moduli of these materials can be represented by linear functions of pressure and temperature.
The isothermal bulk moduli of other observers have been used to compute the equivalent adiabatic moduli by use of the thermodynamic relations. Comparison shows that at high pressures the dynamic moduli agree with the adiabatic moduli within the experimental accuracy for iron and aluminum. At low pressure the dynamic modulus of Armco iron is less than the adiabatic. The deviation is small but is about four times the estimated probable error.
27(1956); http://dx.doi.org/10.1063/1.1722227View Description Hide Description
The diffusion of Au in Ag has been measured in the temperature range from 650 to 950°C, using Au198 as a tracer. The data indicate that the diffusion coefficient is given by the equation D=0.26 exp(−45 500/RT) cm2/sec. The fact that the activation energy for the diffusion of Au in Ag is identical to that for self‐diffusion in Ag is discussed briefly.
27(1956); http://dx.doi.org/10.1063/1.1722228View Description Hide Description
The activation energy of creep was determined for a series of tinsingle crystals oriented with the specimen axis close to a  direction. Two activation energies were found. A value of approximately 22 000 cal/m was obtained for temperatures above 130°C; a value of about 11 000 cal/mole for temperatures below 130°C.
27(1956); http://dx.doi.org/10.1063/1.1722229View Description Hide Description
Precipitates formed by copper diffused into silicon crystals were studied by optical means using an infrared image tube. Linear aggregates are identified as dislocations by correlation with etch pits. Dislocations in as‐grown crystals are usually curvilinear. Dislocation loops formed during plastic deformation consist of straight‐line composite portions and pure screw portions, all of which lie in 〈110〉 directions. Precipitates frequently are not found on screw dislocations; when present they differ in detail from those on composites. Interactions between adjacent dislocations can be seen. Etch pits associated with either composite or screw dislocations are similar in form, indicating that the pits formed in as‐grown crystals by the technique described identify all the dislocations.Dislocation loops enter from the surface upon deformation of crystals with relatively few grown‐indislocations.Dislocation loops formed entirely within the crystal in patterns consistent with operation of the Frank‐Read mechanism are observed in less perfect crystals.
27(1956); http://dx.doi.org/10.1063/1.1722230View Description Hide Description
The Hall coefficient of titanium has been measured over a temperature range from 350°K to 1100°K. The coefficient is −2.0×10−11m3/coulomb near room temperature, reverses sign at (675±30)°K, and increases to a value of +3.5×10−11m3/coulomb at 1100°K. The measurements were made on titanium samples which had a purity of 99.99%. The samples were heated by direct current, and the temperatures were determined indirectly from the resistivitymeasurements which were made with each Hall measurement. The resistivity is 0.48 microhm m at 350°K. It increases to a maximum of 1.76 microhm m at the crystal transition temperature.
27(1956); http://dx.doi.org/10.1063/1.1722231View Description Hide Description
An apparatus has been constructed for the study of deformation under tension of single and polycrystallineice.Deformations down to 10−5 cm could be measured. Deformation of single and polycrystals was investigated as a function of time, stress, and temperature. Whereas the strain rate for polycrystallineice decreases with time, that for single glacierice increases linearly with time. The deformation for fine‐grained polycrystallineice consists of an instantaneous elastic deformation, a transient creep and a steady state creep.Deformation curves can be represented by empirical equations. The recovery curves on removal of the loads have also been investigated and the plastic flow has been deduced from the residual deformation after complete recovery. This plastic flow was found to be Newtonian within the range of stresses investigated and the viscosity coefficients can be represented by an exponential relationship as follows: η1 = 7.5·e +16100/RT poises, where 16 100 calories is the energy of activation for the plastic flow. The total deformation can be represented satisfactorily by a large number of Voigt units representing a distribution of retardation times, in series with a Maxwell unit.
The experimental results are further discussed in the light of current theories of dislocations and tentative mechanisms for the deformation of single and polycrystallineice are proposed.
27(1956); http://dx.doi.org/10.1063/1.1722232View Description Hide Description
The diffusion of water from glass is shown to be a reversible process. The equilibrium partial pressure of water for a soda‐lime glass was 10 mm (Hg) at 500°C and 12 mm (Hg) at 550°C. In a very dry atmosphere the diffusion of water from the glass proceeded as well as in vacuum.