Volume 21, Issue 4, 01 April 1950
Index of content:
21(1950); http://dx.doi.org/10.1063/1.1699651View Description Hide Description
21(1950); http://dx.doi.org/10.1063/1.1699652View Description Hide Description
One factor inherent in most electron microscopes is the magnetic asymmetry (astigmatism) in the objective lens. This is often due to imperfect machining, localized defects (impurities) in the iron, or grain anisotropies. The lapping technique described herein effectively reduces the effect of imperfect machining (misalignment of components). Critical faces and the bores through the pole piece are lapped with a series of powders (alumina) which increase in fineness until the final process is that of polishing. A set of jigs and laps (brass and hard wood) are described; their specifications and use so as to properly alter the critical pole piece surfaces are also described. Electron micrographs illustrating the value of this lapping procedure in the improvement in objective lens symmetry are included.
21(1950); http://dx.doi.org/10.1063/1.1699653View Description Hide Description
The permeability of a number of thin organic films toward oxygen and nitrogen has been measured. For an ethyl cellulose film the studies were extended to include CO2, A, He, and H2. The enrichment of a binary gas mixture in a single stage of permeation has been calculated for the extreme cases of perfect mixing and no mixing. Application of these results indicates that the use of a fractional permeation process may be of practical importance in effecting the separation of oxygen from air, helium from natural gas, and hydrogen from coke‐oven gas, as examples.
D.C. Characteristics of Silicon and Germanium Point Contact Crystal Rectifiers. Part II. The Multicontact Theory21(1950); http://dx.doi.org/10.1063/1.1699654View Description Hide Description
Part I has described the d.c. current‐voltage characteristics obtained for typical Si and Ge crystal rectifiers, has given a summary of the published theories of the rectifier, and has shown that none of these theories accounts for the principal features of the observed characteristics. Part II describes the multicontact theory, an extension of the diode theory, and shows that this theory does account for the behavior of the observed characteristics. There also is described a graphical treatment, based upon the multicontact theory, which may be used in the analysis of experimental characteristics. The multicontact theory assumes that the contact potential at the metal‐semiconductor surface varies from spot to spot. It is shown that an exponential distribution function for contact potentials may be used to account for the observed logarithmic slopes and that the observed temperature behavior of the characteristics may be explained by assuming that the distribution function varies with temperature in a manner dependent upon the nature of the contact.
21(1950); http://dx.doi.org/10.1063/1.1699655View Description Hide Description
An analysis of a sampling servo mechanism with an error‐clamping device and linear forward and return paths is given. The method leads to a determination of the continuous output of the system in terms of quadratures and gives explicitly the value of the output at the discrete sampling instants. Frequently this is all that is needed in order to study the output variation with time. The question of stability is discussed and a criterion for testing the stability of the system is given in terms of the system parameters.
21(1950); http://dx.doi.org/10.1063/1.1699656View Description Hide Description
The variations in velocity of dilatational waves in the pressure range 0 to 15,000 lb./in.2 and temperature range 30° to 90°C have been measured for samples of polystyrene, Lucite, and polyethylene. The velocity of rotational waves was also measured for polystyrene and Lucite. In polyethylene no trace of a rotational wave could be identified. The elastic moduli and Poisson's ratio are computed over the experimental range.
21(1950); http://dx.doi.org/10.1063/1.1699657View Description Hide Description
A method for the summation of certain types of Fourier series by the use of Laplace transforms is presented. The procedure enables one to easily obtain graphical representations of functions defined by Fourier series in many cases of practical importance.
21(1950); http://dx.doi.org/10.1063/1.1699658View Description Hide Description
It is shown that when certain plausible assumptions are fulfilled simple scaling laws govern the times required to produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical geometrically except for a difference of scale. It is suggested that experimental studies of the effect of such a change of scale may prove valuable in identifying the predominant mechanism responsible for sintering under any particular set of conditions, and may also help to decide certain fundamental questions in fields such as creep and crystal growth.
21(1950); http://dx.doi.org/10.1063/1.1699659View Description Hide Description
A resonant bar method for obtaining the dynamic elastic properties of a thin cement layer is discussed. Formulas are deduced for the calculation of Young's modulus from the measured resonant frequency and band width of two similar steel bars, cemented end‐to‐end. The accuracy of the method is discussed.
21(1950); http://dx.doi.org/10.1063/1.1699660View Description Hide Description
An electronic apparatus which assists in the finding of the roots of polynomials is described. The device accepts the coefficients of polynomials of degree 10 or less and yields the absolute magnitude of the roots to a precision of 0.01 relative to the largest coefficient in the polynomial and an angular accuracy of 0.3 degree on the angle of the root.
The method used is the electronic analog of the expressionSinnθ and cosnθ are generated by a motor‐driven commutator. The radius r is controlled manually. Hence a plot of ω is placed on the oscilloscope for a fixed r as a function of θ. To find the root r is manipulated until ω crosses the origin in the ω‐plane. The value of θ is found by inserting a pip on the intensity of the oscilloscope under a variable phase control. When this pip is moved to the origin of the ω‐plane the phase meter indicates the angle of the root in the z plane.
21(1950); http://dx.doi.org/10.1063/1.1699661View Description Hide Description
The CdS x‐ray detector is compared with the photo‐multiplier x‐ray detector. Reduced to the same solid angle the sensitivity of the CdS x‐ray detector is of the same order of magnitude as the sensitivity of the photo‐multiplier x‐ray detector. As the internal amplification of the primarily released electron currents in CdS is of the order 103 to 104 compared with the amplification in the multiplier tube of ½×106 the conversion process of x‐ray quanta in slow electrons in CdS must be more efficient than the corresponding processes in the combination crystal phosphor screen photo‐multiplier. Calibration curves show a linear relation between photo‐current and intensity of x‐rays at low irradiation levels. At high irradiation saturation effects occur. Oscilloscope curves of the response at 60 x‐ray pulses per second are given. At strong irradiation the photo‐current over a long time increases in a completely reversible way. X‐ray intensities as low as 150 quanta CuK α per second are registered with a circuit employing a WL 759 trigger tube.
21(1950); http://dx.doi.org/10.1063/1.1699662View Description Hide Description
The diffraction of a plane electromagnetic wave by an infinitely long, perfectly conducting cylinder has been treated by a variational method (see the two papers by H. Levine and J. Schwinger). The incident field is assumed to be polarized in the direction of the cylinder axis, and thus the entire field is of two‐dimensional nature. This formulation yields an expression for the diffracted cylindrical wave amplitude, at distances from the cylinder large compared to its transverse dimension and the wave‐length, which is stationary relative to small independent variations of the surface currents arising from plane‐wave excitation along a pair of directions in space; furthermore, the stationary form of the diffracted amplitude is independent of the scale of the surface currents. In accordance with a theorem of Levine and Schwinger, the total plane‐wave scattering cross section is simply related to the diffracted cylindrical wave amplitude in the direction of incidence. To examine the high frequency behavior of the cross section, the surface current induced by a plane wave is taken different from zero only on the illuminated part of the cylinder, where its value is derived from the tangential component of the incident magnetic field. The resulting cross section is obtained and is shown to approach 4a when ka approaches infinity (k=2π÷wave‐length, a equals the radius of cylinder).
21(1950); http://dx.doi.org/10.1063/1.1699663View Description Hide Description
It is suggested that when the air temperature increases upward an efficient and economical method for modifying low temperatures near the ground is to force air downward through vertical stacks from the relatively warm air aloft. Theory is developed which shows that the necessary power per unit mass is mainly a function of the air velocity in the stack. The energy required by this method is compared with the energy required to heat the air originally near the ground, and it is shown that under a strong temperature inversion the power method requires considerably less energy than the heating method.
Mode Conversion Losses in Transmission of Circular Electric Waves through Slightly Non‐Cylindrical Guides21(1950); http://dx.doi.org/10.1063/1.1699664View Description Hide Description
A general expression is derived for the effective attenuation of circular electric (TE 01) waves owing to mode conversions in a section of wave guide whose shape deviates slightly in any specified manner from a perfect circular cylinder. Numerical predictions are in good agreement with experiment for the special case of transmission through an elliptically deformed section of pipe. The effect of random distortions in a long wave guide line is analyzed in terms of the mean‐square values of the Fourier coefficients describing the distortions; and from the general formulas it appears that the mode conversion losses depend in large measure upon the statistical distribution of the various types of distortion. Under certain rather arbitrary simplifying assumptions about this distribution, it is calculated that mode conversions in a 4.732‐inch copper pipe whose radius deviates by 1 mil r.m.s. from that of an average cylinder will increase the attenuation of the TE 01 mode at 3.2 cm by an amount equal to 8 percent of the theoretical copper losses. The dependence on frequency of mode conversion losses in such a guide is discussed.
21(1950); http://dx.doi.org/10.1063/1.1699665View Description Hide Description
The wettability of crystals, glasses, and even of water itself can be temporarily decreased by bringing ions of high polarizability into their surfaces. Base exchange experiments are described where the hydrogen ions present in the surface layers of bentonite (a clay mineral of high exchange capacity) and of a soda‐lime glass are replaced by different cations. This substitution seems to have no particular effect on the hydrophilic and rheological properties of the carriers as long as their surfaces are kept in contact with water. After drying, however, the surfaces which contain ions of high polarizability become hydrophobic, at least temporarily. A porous clay film will no longer absorb water instantaneously after having been treated with Ni2+, Mn2+, Hg2+, or similar ions with incomplete outer electron shells. Contact angles with water up to 70° could be observed for a short period for Hg2+ and Pb2+ clays.
Glass capillary tubes which have been treated with non‐noble gas‐type ions show a capillary rise which is much smaller than that observed with the tubes which have been treated with HCl and water only. This depression of the capillary rise, too, is temporary and can be observed only if the glass wall has been thoroughly dried previous to the experiment and if the capillary rise is measured in the upward direction.
An explanation is presented for these and allied phenomena on the basis of the polarization of ions in the strongly asymmetrical forcefields of interfaces. The experiments are correlated with the hysteresis of the contact angle and with observations concerning adhesion phenomena and catalytic activities of heavy metal ions at interfaces.
21(1950); http://dx.doi.org/10.1063/1.1699666View Description Hide Description
Measurements are reported on the distribution of ionization in depth of aluminum produced by steady beams of two and three million‐volt electrons. The variation of cathode‐ray current density in a plane transverse to the beam, the effect on this transverse distribution of additional aluminumscattering foils, and a practical method of cathode‐ray dosage computation are given.