摘要:

The reason for contact differences of potential is discussed. Since it is purely a surface phenomenon, subject to wide variation as the result of adsorbed and chemically combined surface layers, it is of interest to observe the change of contact potential as a function of time for various freshly evaporated metal films in high vacuums. The results thus obtained indicate characteristically different behavior for each of the six metals studied. Electrical resistance measurements made on similar metal films under similar conditions are correlated with contact potential measurements.

ISSN:0021-8979    

DOI:10.1063/1.1699880

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Volta potentials of various metals approximately parallel the emf series. Metals abraded wet are more noble than when abraded dry, the transition metals changing an average of 0.28 volt compared with 0.07 volt for the nontransition metals. This owes possibly to the greater tendency of the transition metals to chemisorb oxygen, especially in the presence of moisture. All metals change in the noble direction upon aging in air, the average value being 0.3 volt after six days.Potentials for copper‐nickel alloys, plotted with composition, fall on a sigmoid curve, whereas potentials for composite electrodes made up of separate nickel and copper areas are linear with percent area of nickel exposed. Commercial nickel‐copper alloys above 50 percent nickel age more rapidly than laboratory alloys. The cause may lie in deoxidizers like magnesium or aluminum, present in commercial alloys, but absent in the laboratory alloys. This suggests a nondestructive method for detecting impurities of this kind in metals.

ISSN:0021-8979    

DOI:10.1063/1.1699881

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

A technique for preparing autoradiographs of metals and minerals on a microscale is described. Limitations of this technique and possible variations are indicated. Illustrative examples of autoradiographs are presented and interpreted.

ISSN:0021-8979    

DOI:10.1063/1.1699882

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The problem under discussion is the velocity of ultrasonics in liquids, specifically water, at various temperatures and high pressures. This report deals with the apparatus, and with some representative results obtainable with it.The electrical apparatus makes use of a presentation unit (Dumont 256 or A/R 'scope) which synchronizes the sequence of operations by internally generated trigger signals at low repetition rates. Three‐ to five‐microsecond pulses at controllable amplitude and approximately 15 Mc/sec carrier frequency are imposed on an X‐cut quartz crystal which has been placed inside a pressure‐supporting vessel containing the test sample. The resulting ultrasonic pulses in the liquid are reflected back and forth over a definite path length between the transducer and a mirror. After amplification in a receiver strip the complete echo‐pattern becomes available for visual comparison on the presentation screen. The timed relative delay (and the measurable amplitude differences) between successive pulses furnish data for velocity (and attenuation) measurements when certain corrections are attended to.Representative data are given for the velocity in water (to 0.1 percent) as a function of pressure to about 6000 atmospheres. From the data on water, information is derived on the temperature coefficient of sound velocity and on the ratio of specific heats at increasing pressures. The latter results are compared with those obtained directly from purely static experiments, and the advantages of the ultrasonic measurement technique for the derivation of some of the thermodynamic coefficients are cited.

ISSN:0021-8979    

DOI:10.1063/1.1699883

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

Theory is presented for oblique shock waves involving (1) heat exchange,k1, (2) transfer of part of the compressible fluid,k2, to an incompressible state, and (3) change in specific heat ratios,k3. By matching mass flow, momentum, and energy relations across an oblique finite disturbance it is shown that these additional conditions introduce new terms in the solution of the formkiu1/(u1−u2). Forkinot zero a minimum shock angle is found which always exceeds the Mach angle by a definite increment. For air‐flow atu1/a* of 1.5 with dew point −20°F this increment is calculated as 3.1 degrees. For expansion about a corner such flow differs from the Prandl‐Meyer type and in supersonic nozzles, for example, oblique shock waves are predicted which can explain anomalous pressure losses, the presence of extraneous waves, and non‐isentropic effects. Hodographs of the velocity solution include a triply infinite set of curves, one, the usual set, introduced by Mach number as parameter and the other two byki. The theory includes physically possible normal and oblique flows involving increase of supersonic velocities and subsonic velocities of which oblique flame fronts are indicated as one form. Applied to air condensations in hypersonic flows isentropic disturbances having appreciable angular flow shift and stagnation pressure losses are predicted.

ISSN:0021-8979    

DOI:10.1063/1.1699884

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

In his paper, the author determines the stability of periodic solutions of (1) the general forced Duffing equation without damping,x¨+x+&egr;f(x)=&egr;E cos&ohgr;t,f(x) a polynomial, &egr;≪1; (2) the forced Duffing equation with damping,x¨+&egr;rx˙+&agr;x+&egr;x3=&egr;E cos(&ohgr;t+&psgr;), &egr;≪1; and (3) the general forced Van der Pol equation,x¨−&egr;f(x˙)+x=&egr;E cos(&ohgr;t+&psgr;),f(x˙) an odd polynomial, &egr;≪1, by seeking conditions necessary to insure periodic or almost‐periodic solutions of the corresponding variational equation. The former are obtained rigorously by means of an existence proof and the latter formally by perturbation series. A vertical tangent theorem is derived which states that the locus of the points of contact of the vertical tangents to the response curves is a stability boundary, since it coincides, in the first approximation, with the locus of periodic solutions of the variational equation. These techniques are illustrated by considering the forced vibrations of a triode oscillator with a fifth‐order tube characteristic, this being a particular case of (3), where we setf(x˙)=x˙−13x˙3+&lgr;x˙5.

ISSN:0021-8979    

DOI:10.1063/1.1699885

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

From Maxwell's equations, an expression for the complex power associated with a wire circuit is formulated and broken into a complex input power and a complex power into the external fields associated with the circuit, the latter including the radiated power. From these powers, the internal and external impedances of the circuit are obtained such that the current is not required to be everywhere in time phase within the circuit. This concept is extended to coupled circuits, bringing out some of the relations between some conventional methods for obtaining the driving point impedance of antenna arrays. The theory does not require the current distributions to be postulated, but in practical applications such a postulate becomes necessary unless the solution is obtained by a method such as the integral equation method. The resulting circuitry may readily be reduced to that for lumped elements. A more critical study of the impedance formulas is given in the appendix, based upon the reciprocity theorem which is derived therein.

ISSN:0021-8979    

DOI:10.1063/1.1699886

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

In this paper the author discusses the heat transfer by free convection from electrically heated wires (copper and iron) to distilled water between freezing and boiling, to water boiling at atmospheric conditions and to air at room temperature. The wire surface temperatures needed for the heat transfer determination are calculated from the theoretical temperature distribution in the wire. The experimental results show maximum heat transfer rates of about 2,200,000 Btu/hr, ft2near freezing and 450,000 Btu/hr, ft2near boiling. For boiling water, as the temperature difference between the wire surface and water is increased, the heat transfer coefficient first increases reaching a maximum, then decreases reaching a minimum and increases again (because of radiation) until the wire fails by melting. For copper the maximum film coefficient of 9100 Btu/hr, ft2, °F is observed at 49 degrees temperature difference and the minimum of 136 Btu/hr, ft2, °F at a temperature difference of 690 degrees F. Corresponding values for iron are 12,300 Btu/hr, ft2, °F at a 33 degree temperature difference and 142 Btu/hr, ft2, °F at a 620 degree difference. When air at room temperature surrounds the wire the heat transfer coefficient for copper with an oxide film varies from 0 to 30 and for iron from 0 to 26 Btu/hr, ft2, °F as the surface temperature of the wires is raised until failure occurs.

ISSN:0021-8979    

DOI:10.1063/1.1699887

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

A mathematical expression is derived for computing dimensions of a liquid helium cryostat of specified performance characteristics. Use is made of a finite difference summation scheme applied to a simple thermomechanical model, usingTas the independent variable, for a liquid evaporating in a container with good exchange to the walls. The scheme thus takes into account the heat capacity of the gas between the boiling point and the upper cryostat temperature. The results obtained are compatible with the actual performance of cryostats in operation.

ISSN:0021-8979    

DOI:10.1063/1.1699888

出版商:AIP    

年代:1951

数据来源: AIP

摘要:

The preparation of various replicas of a fused iron catalyst is described. Micrographs of these replicas show comparable fine structures that are attributed to the pores developed in the catalyst upon reduction. Dimensions of these fine structures compare reasonably well with pore diameters calculated from pore volume‐surface area determinations.

ISSN:0021-8979    

DOI:10.1063/1.1699889

出版商:AIP    

年代:1951

数据来源: AIP