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In classical aerodynamics the air is considered as a continuum and its flow characteristics are described by the equations of fluid mechanics. From the standpoint of kinetic theory, the validity of these equations is based on the physical assumption that the behavior of the fluid is determined almost exclusively by the interactions between the individual molecules and only to a very minor, generally negligible, ex tent by the interaction between molecules and solid boundaries. Another expression of this physical situation is contained in the statement that under the conditions encountered in classical fluid mechanics, the mean free paths between individual molecules is very small compared with the dimensions of the bodies in contact with the fluid. If, however, the density of the gas is gradually being decreased to reach such values as, for instance, exist in tne upper atmosphere, the mean free path which is inversely proportional to the density will gradually increase until the condition mentioned above is no longer valid. We are then reaching the regime known as rarefied gas dynamics, which is characterized by a mean free path λ between the molecules of the gas of the same order or even larger than the critical dimension d of the system. The ratio between this mean free path λ and the characteristic dimension K=λ/d, commonly called the Knudsen number, divides gas dynamics into the various flow regimes, values of K 10 free molecular flow, and intermediate values the re gions commonly called slip and transition flow. In this series of lectures we will be concerned only with the regions of very large K's, that is with free molecular flow, also called Knudsen flow. In this flow regime individual molecules will make many collisions with the solid walls bordering on the system before they will have a chance to collide with one another. As a result, the flow characteristics are determined by the interactions between solid surfaces and gas molecules while the continuum quantities such as viscosity and heat conductivity lose their importance.

Nach den heutigen Anschauungen sind die Atome und Molekule aufgebaut aus den beiden Elementarbausteinen der Materie, den positiv elektrischen Protonen und den negativ elektrischen Elektronen. Die Untersuchung der Anordnung dieser Elementarteilchen im Atom und Molekul ist eine wesentliche Aufgabe der modernen Physik. Fur die Atome ist dieses Problem durch die neuesten Fortschritte der Quantentheorie in Verbindung mit zahlreichen Experimenten schon weitgehend gelost. Wenn auch die Quantenmechanik im Gegensatz zur alten Atomtheorie darauf verzichten mus, die Bahnen der Elektronen im Atom raumzeitlich zu beschreiben, so gibt sie doch statistisch die durchschnittliche Verteilung der positiven und negativen Ladungen im Atom an. In allen bisher untersuchten Fallen hat es sich ergeben, das beim normalen Atom die Schwerpunkte der positiven und negativen Ladungen zusammenfallen, so das, da auch der Betrag der positiven und negativen Ladungen der gleiche ist, das Atom nach ausen hin wie ein elektrisch neutraler Korper wirkt.

Dr. Immanuel Estermann was engaged in writing a book on the History of the Molecular Beam Method when he died in Haifa, Israel on 31 March 1973. The original plan of the manuscript was to cover three distinct historical periods: the first, the years 1911–1933, when the center of research in molecular beams was at the University of Hamburg; the second, from 1933 to the outbreak of World War II, when the most active laboratory working in the field was at Columbia University; and the postwar period, when many laboratories on both sides of the Atlantic became actively engaged in this field. The material in this paper deals primarily with the early historical period and would have formed the essence of the first two chapters of the book. In addition to presenting interesting historical facts on a crucial period in the development of quantum physics, it contains some amusing historical sidelights on the research personalities that dominated that period.

The magnetic moment of the proton was measured by the method of the magnetic deflection of molecular beams employing ${\mathrm{H}}_{2}$ and HD. The result is ${\ensuremath{\mu}}_{P}=2.46{\ensuremath{\mu}}_{0}\ifmmode\pm\else\textpm\fi{}3$ percent.

The free fall of atoms is observed in long molecular beams of potassium and cesium atoms. The measurement of the intensity distribution in a beam deflected by gravity represents the velocity distribution of the beam atoms and permits an accurate determination of this distribution. The results show that the measured values agree in general with those calculated for a modified Maxwellian velocity distribution in the beam. At larger deflections, i.e., for slow atoms, a deficiency of intensity was observed, which increased both with increasing deflection and with increasing pressure in the oven where the beam originates. This deficiency is explained by collisions in the immediate vicinity of the oven slit.

A comparison of the method for determining electric moments of molecules by deflection of molecular beams by inhomogeneous electric fields with the dielectric constant method shows that the molecular beam method has certain advantages. This method determines the electric moment of gases directly and is able to detect the effect of higher rotational states of non‐gyroscopic molecules. Moreover, a deviation of the axis of the dipole moment from that of the axis of rotation by other than 90° should be detected. The additional energy due to an electric field is proportional to the square of the field intensity and is a function of the quantum numbers m and j, if the electric moment is perpendicular to the axis of rotation. Otherwise there will be a linear dependence on the intensity of the electric field. An experimental method of the Rabi type for studying the HCl molecule is described. The patterns produced by the undeflected and deflected molecules are given and from these patterns the electric moment of the HCl molecule is found to be approximately 1.95×10—18 e.s.u.

Trifft ein Molekularstrahl (H2; He) auf eine Kristallspaltflache (Li F) auf, so zeigen die von ihr gestreuten Strahlen in allen Einzelheiten eine Intensitatsverteilung, wie sie den von einem Kreuzgitter entworfenen Spektren entspricht. Die aus der Gitterkonstante des Kristalls berechnete Wellenlange hat fur verschiedene m und υ den von de Broglie geforderten Wert λ=h/m · v.

Es wird eine neue Methode zur quantitativen Intensitatsmessung von Molekularstrahlen vorgeschlagen, die darauf beruht, das die durch Ionisierung der Molekule erzeugten Ionen die Raumladung um einen Gluhdraht zerstoren und den Elektronenstrom steigern (Kingdon). Die Brauchbarkeit dieser allgemein verwendbaren und sehr empfindlichen Methode wird durch orientierende Vorversuche gezeigt.

The program of this symposium, which was held in Paris 26–29 June, 1962, included papers on new developments in the theory of flow of neutral and partly ionized gases at low densities, interactions between gas molecules and solid surfaces, high-speed and thermal molecular beams, measurements of drag and heat transfer in the transition and free molecule flow regime, and new experimental techniques. About 25 of the 60 papers presented are discussed in the following pages; the complete program is reproduced as Appendix I.

Alkali halide crystals become colored when irradiated with x-rays. Theoretical considerations indicate that the coloration is due to absorption centers which consist of electrons trapped at negative ion vacancies. Precision measurements of the density of uncolored crystals, however, indicate that it is very unlikely that there is a sufficient number of vacancies present in well annealed crystals to account for the number of color centers, which may be determined optically, unless vacancies are formed as a result of the irradiation.

With a view to studying the mechanism of heat conduction in low conductivity alloys, a method has been devised by which the thermal conductivity of relatively small samples (⅛‐ to ¼‐inch diameter, 1 to 2 inches long) of various materials can be measured in the temperature regions obtainable with liquid nitrogen, liquid hydrogen, and liquid helium.Preliminary measurements on several commercial alloys (monel, inconel, and stainless steel) gave Wiedemann‐Franz ratios several times greater than the theoretical value of 2.45×10−8 watt‐ohm/deg2, the deviation being greater for annealed than for cold‐worked specimens. This has been interpreted in terms of appreciable lattice conduction of heat in these alloys.Following these results, samples of an alloy of 90 percent copper 10 percent nickel were prepared with varying amounts of cold‐work and with different grain sizes. Results with these samples were similar to those obtained with monel and inconel and confirm the hypothesis of lattice conduction. They also giv...

Strahlen aus Wasserstoffmolekulen wurden nach der Methode von Gerlach und Stern magnetisch abgelenkt und so ihr magnetisches Moment bestimmt. Die Messungen an Parawasserstoff ergaben das von der Rotation des Molekuls herruhrende magnetische Moment zu etwa 1 Kernmagneton (1/1840 Bohrmaneton) pro Rotationsquant. Die Messungen an Orthowasserstoff ergaben das magnetische Moment des Protons zu 2 bis 3 Kernmagnetonen (nicht 1 Kernmagneton, wie bisher vermutet wurde).

Measurements of the electrical resistivity of thin silver films prepared by evaporation on amorphous substrates in ultrahigh vacuum indicate partial specular reflection of the conduction electrons at the films' surfaces. Some correlation between the electrical resistivity and the structure of the deposited films was observed, and the influence of residual gases during the evaporation process on structure and properties of the films was investigated.

High precision measurements of the specific heat of germanium were carried out in the temperature region between 20°K and 200°K. Three samples of germanium were investigated: an extremely pure sample, one of intermediate purity (1018 impurity centers/cm3), and one with an addition of 0.006 atomic percent aluminum (2×1020 impurity centers/cm3). No anomaly such as reported by Cristescu and Simon was found in any of the samples. However, a small dependence of the specific heat on the concentration of impurity centers was observed.

Die de Broglie-Wellen wurden auf zwei Wegen monochromasiert: 1. Ein gewohnlicher Molekularstrahl (mit Maxwellverteilung der Geschwindigkeiten) von Heliumatomen wurde an einer LiF-Spaltflache gebeugt; aus dem Beugungs-spektrum wurden Strahlen bestimmter Richtung, also Wellenlange, ausgeblendet und die erfolgte Monochromasierung durch Beugung an einem zweiten Kristall nachgewiesen. 2. Ein gewohnlicher Molekularstrahl wurde durch ein Zahnrad-system geschickt, das nur Atome eines bestimmten Geschwindigkeitsbereichs passieren lies, und an einer LiF-Spaltflache gebeugt. Die so gemessene Wellenlange stimmte mit der aus der—grobmechanisch bestimmten — Geschwindigkeit berechneten (λ=h/mv) auf 1% uberein.

The mean free paths of cesium atoms in helium, nitrogen, and cesium vapor were measured with a molecular beam apparatus permitting the measurement of scattering deflection of about 5 seconds of arc. Utilizing the free fall of the beam atoms, the variation of the mean was found to be in agreement with the equations calculated on the basis of classical theory treating the atoms as elastic spheres. The measured values of the sums of the effective atomic radii are 12.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ cm for Cs-He, 17.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ for Cs-${\mathrm{N}}_{2}$, and 27.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ cm for Cs-Cs collisions. These values correspond to mean free paths of 2.1 meters in He, 2.2 meters in ${\mathrm{N}}_{2}$, and 1.36 meters in Cs-vapor of ${10}^{\ensuremath{-}6}$-mm Hg pressure for cesium atoms with a velocity corresponding to the maximum intensity of the gravity deflection curve.

By using a ``twisted tube'' heat interchanger, it has been possible to build a small hydrogen liquefier of great simplicity and efficiency, which can be operated from commercial hydrogen cylinders without the use of a compressor and delivers about one liter of liquid hydrogen per hour.

The specific heats of magnesium, titanium, zirconium, and chromium have been measured between 1.8\ifmmode^\circ\else\textdegree\fi{} and 4.2\ifmmode^\circ\else\textdegree\fi{}K by the vacuum calorimeter method. In each case it has been possible to represent the results by an expression containing an electronic term linear in the temperature and a lattice term cubic in the temperature. The electronic contribution to the specific heats of these metals is in reasonable agreement with expectations based on the Sommerfeld theory and, in particular, with the results of more recent detailed calculations of energy bands in transition metals.This work has been facilitated by the development of a sensitive semiconducting resistance thermometer of germanium-indium alloy for use at liquid helium temperatures.

A torsion balance, designed to measure the force on a flat plate (or other simple geometry) in a low density gas flow, is described. The balance has been used with two interchangeable torsion member sizes, with diameters of 0.00064″ and 0.00090″, which measured forces up to a maximum of 0.6 milligram and 2.4 milligrams, respectively. Calibrations (obtained in still air using weights) were reproducible within ±0.005 mg and ±0.04 mg for the two sizes. Data obtained for a flat plate in a gas stream under large molecular mean free path conditions are presented.

The design principles and operating characteristics of residual gas analysers and their performance in an unbaked high-vacuum system are discussed.

THE spin of the electron has the value and its magnetic moment has the value, or 1 Bohr magneton. The spin of the proton has the same value,, as that of the electron. Thus for the magnetic moment of the proton the value =1/1840 Bohr magneton = 1 nuclear magneton is to be expected.

IN a previous note1 we reported, together with Mr. Frisch, on experiments concerning the deflection of a beam of ‘ordinary’ hydrogen molecules in an inhomogeneous magnetic field. From these experiments, we were able to derive the magnetic moment of the proton. The value obtained was 2·5 nuclear magnetons (not 1, as expected theoretically).