Index of content:
Volume 20, Issue 4, 01 April 1949
Investigations of a Bearing for Small Angular Deflections without Any Friction and with Negligible Restoring Torque20(1949); http://dx.doi.org/10.1063/1.1698360View Description Hide Description
This paper deals with bearings supported on metal beams under mechanical stress in the axial direction. Because of a phenomenon related to the bending of long columns under axial compression the torque of the total arrangement can be made zero or even negative. Such a bearing can be designed with two decisive advantages, absolutely no friction, and practically no restoring torque as far as small angles of rotation are concerned. Two different types of beam bearings are treated, and their equations for precalculation are derived. Furthermore, the influences of temperature, external forces, and the angle of rotation on the torque of the bearing are investigated. Finally, a combination of two different beam bearings is shown to offer a possible compensation for these unfavorable effects.
20(1949); http://dx.doi.org/10.1063/1.1698361View Description Hide Description
For a definite Mach number, given by M 2 = 4/(3−γ), all Mach lines in a steady plane two‐dimensional potential flow of a perfect gas with expansion exponent γ have zero curvature. The use and significance of this relation are discussed, and the hypothetical expansion law consistent with straight Mach lines is deduced.
20(1949); http://dx.doi.org/10.1063/1.1698362View Description Hide Description
The structure of thin metallic films was studied by means of electron diffraction and electron microscopy. Microcrystal size can be correlated with the melting point of the metals, those with high melting points producing continuous films of small unoriented microcrystals while those with low melting points produce large microcrystals oriented preferentially with respect to the substrate. The effects of varying experimental conditions on the filmstructure of selected metals are reported. The electron beam of the electron microscope affects metals with low melting points most directly, producing melting, sublimation or crystallization. Variations in the velocity of the impinging atom, obtained by passing thermal atoms through a mechanical velocity selector, produce no effect for metals whose vapor is monatomic. Antimony whose vapor is found to consist of polyatomic molecules as well as atoms produces films whose grain size seems to vary with the size of the molecules forming the antimonyfilm. The rate of evaporation merely determines whether zinc or cadmium films may or may not be formed while for antimonyfilms it also determines the type of the films formed. Those formed by rapid evaporation consist of small crystallites while those formed by slow evaporation consist of large amorphous patches. This effect is explained on basis of the mobility of the atoms on the substrate, the mobility time being determined to be less than 2×10−5 second. The mobility time and therefore the structure depends upon the type of substrate. The degree of vacuum is critical when the residual vapor reacts with the film being formed, but if no reaction takes place between residual vapor and film, pronounced variation for films formed in a vacuum are produced only when the pressure is above 10−2 mm of mercury.
20(1949); http://dx.doi.org/10.1063/1.1698363View Description Hide Description
A method is described for determining the variation of the resistance (R) and the reactance (X) of an electrical network from mechanical plots of the numerators and denominators (both functions of the angular frequency ω) occurring in the expressions for R and X. From the expressions, R = N(ω)/D(ω) and X = N′(ω)/D(ω), mechanical plots of N, N′, and D versus ω can be obtained with a mechanical harmonic synthesizer, and the values of R or X corresponding to any value of ω may then be determined by taking the ratios of the ordinates of N and D, or N′ and D, corresponding to the desired value of ω.
20(1949); http://dx.doi.org/10.1063/1.1698364View Description Hide Description
The theory of the hardness is reviewed and measurements are made with respect to the application of hardness data to contact problems, particularly for the determination of the real contact area. The ball indentation method is recommended, with hardnessH defined as the ratio P/Sm between the contact load P and the mouth area Sm of the indentation. This hardness is not a single valued material constant but is a function of the specific depth D of the indentation, where D is the ratio between the actual depth and the radius of curvature of the indentation. The variation of H with geometric and metallurgical conditions is discussed together with some empirical formulae to describe the experimental results. The relation between the hardness and the yield point is discussed. A comparison with Brinell and Rockwell hardness numbers is also made. The indentation which two crossed cylindrical rods produce in each other is found to give nearly the same hardness as the ball indentation tests if D exceeds a certain value.
20(1949); http://dx.doi.org/10.1063/1.1698365View Description Hide Description
Short cylindrical antennas have been loaded to resonate at frequencies lower than their natural frequencies by means of dielectric sheaths of cylindrical and conical form. These loadings are compared experimentally with loading by means of metal disks at the ends of the antennas. Resistances at resonance calculated by the Poynting vector method for two assumed current distributions are compared with the experimental results. It is shown that for a given current distribution at resonance, the antenna radiation resistance depends only on the antenna length in wave‐lengths. Particular loading methods alter the current distributions. Those which produce more nearly uniform current distributions yield higher radiation resistances.
20(1949); http://dx.doi.org/10.1063/1.1698366View Description Hide Description
A method is described for measuring the amplitude distribution of high frequency current along some simple types of antennas.Measured curves for some cases are given and compared with some frequently assumed curves. Some of the measured distributions were integrated numerically to determine the driving point resistances. The driving point resistances were measured for geometrically similar antennas in some cases. Deviations from usually assumed curves are significant, but do not bring about large errors in the calculated resistances.
20(1949); http://dx.doi.org/10.1063/1.1698367View Description Hide Description
Signal‐to‐noise ratios at the output of an f‐m receiver are determined as functions of the input signal‐to‐noise ratio, clipping level, and i‐f and audio filter characteristics when random (fluctuation) noise accompanies the signal to be observed. Both narrow and broad band f‐m are examined. Specific calculations are made for sinusoidal frequency modulation, and it is indicated how data for this simple type of signal may be related to the more complex phenomenon of speech. The concept of the signal‐to‐noise ratio is redefined to describe more accurately the sensing of a signal in noise, since the conventional definition for f‐m proves inadequate and misleading, except for strong carriers. Extensive comparisons are made with a‐m reception employing a half‐wave linear rectifier, and it is found that for moderate and weak (<3 db) signals a‐m requires less input signal power against the same noise background than does f‐m to achieve the same desired output. This is particularly evident for weak signals. Only when the signal is strong (>10 db) is f‐m superior to a‐m; here broad band f‐m with very heavy limiting is needed. Narrow band f‐m at its best (no limiting) is comparable with or inferior to a‐m at high signal levels. Limiting proves to be detrimental in narrow‐band operation for all cases involving fluctuation noise, whereas heavy limiting is essential for successful broad‐band performance. Furthermore, the shape of the i‐f response becomes important in the latter, while it is not in the former. A large number of curves illustrating the average and mean‐square signal and noise outputs, etc., and signal‐to‐noise ratios for various conditions of operation are included.
20(1949); http://dx.doi.org/10.1063/1.1698368View Description Hide Description
A theoretical investigation has been made of the power dissipation by eddy currents in a metallic surface at microwave frequencies in the presence of regular parallel grooves or scratches whose dimensions are comparable to the eddy current skin depth. The eddy current equation has been integrated numerically for grooves of various shapes and sizes transverse to the direction of induced current flow, and the corresponding losses are calculated and plotted. The power dissipation is increased by about 60 percent over its value for a smooth surface when the root‐mean‐square deviation of the grooved surface from an average plane is equal to the skin depth; the exact shape of the grooves is not critical. The increase in eddy current losses caused by grooves parallel to the current is shown in a particular case to be only about one‐third as great as the increase caused by transverse grooves of similar size. The effect on losses of an isolated narrow crack or fissure transverse to the current is briefly discussed.
20(1949); http://dx.doi.org/10.1063/1.1698370View Description Hide Description
The characteristics of impulse corona have been studied in a point‐to‐plane gap. Both positive and negative square voltage pulses of one‐ and two‐microsecond duration were used with various voltages and pulse repetition rates. The impulse corona in air is found to be quite similar to the d.c. corona, except that the phenomena are exaggerated by the relative absence of space charge inhibition. At higher voltages, the negative impulse corona was found to have a ``spike'' or streamer type of discharge superimposed on the regular Trichel pulse corona. By using the nonelectron‐attaching gases nitrogen and hydrogen, it is shown that these spikes are due to the negative ion space charge formed by the Trichel pulse in the gap. When the point is grounded, the intense field developed between this negative space charge and the positive space charge at the point is sufficient to cause a streamer to propagate.
20(1949); http://dx.doi.org/10.1063/1.1698371View Description Hide Description
Reference is made to the conjectural nature of our present views on the mechanism of extreme‐pressure lubrication. A differentiation is made between sliding and tearing so that sliding is defined as the relative motion of two solid bodies in frictional contact under the action of a tangential force with the frictional force increasing in direct proportion with the normal load. On the other hand, when the surfaces of the friction elements suffer macroscopic damage during relative motion, the frictional force increases at a more than proportionate rate with the normal load and a case of tearing is established.
The general requirement made of a lubricant is the reduction of the frictional force. The requirement made of an extreme‐pressure lubricant is that it should reduce the frictional force at a sufficiently high value of the normal load for a pressure of at least 1.5×104 kg‐wt./cm2 to apply throughout the run, and in addition, the extreme‐pressure lubricant should prevent tearing or scoring of the surfaces at these comparatively high bearing pressures.
A loaded revolving steel ball in frictional contact with a hardened steel block is a model of a bearing requiring extreme‐pressure lubrication. The temperature elevation at the rubbing contact should be a measure of the energy dissipated by friction, and this can be determined by connecting the rubbing contact to a galvanometer which records the thermal electromotive force generated at the contact. The record of the electromotive force represents the entire frictional history of the contact in every detail. Evidence was found that extreme‐pressure addition agents reduced the frictional force and extended the region of normal loads over which the frictional force was a direct proportion of the normal load.
Various types of extreme‐pressure addition agents gave results which suggested the existence of an optimum dope concentration. In general, over the region of normal loads over which Amontons' law of the direct proportion between the frictional force and the normal load applied, lubricants which reduced the thermal electromotive forces generated at the rubbing contact also reduced the wear diameter. On the other hand, when tearing took place no such parallelism between friction and wear became apparent since the final wear diameter is the integrated result of the wear processes occurring during one run, whereas the record of the electromotive forces gives the details of the change of the frictional force in the course of such a run.
It is suggested that the optimum dope concentration may be a function of two counteracting effects. On the one hand, the extreme‐pressure addition agent is likely to decrease the dielectric breakdown field strength of the thin layer of lubricant and to reduce thereby the electrostatic component of the frictional force. On the other hand, the lubricant becomes more corrosive with increasing concentration of the extreme‐pressure addition agents. As soon as appreciable corrosion takes place, tearing sets in and causes the frictional force to increase. Thus a balance will be struck between the reduction of the frictional force by diminishing its electrostatic component, and an increase of the frictional force owing to the corrosive action of the extreme‐pressure addition agents.
Evidence was found to support this latter view when metallic films were wiped out by hand on metal blocks. It was then observed that under ``dry'' conditions the metallic films tended to reduce the thermal electromotive forces and that the films were more effective in reducing these forces in the order tin,copper,lead. On the other hand, in the presence of an extreme‐pressure lubricant of optimum dope concentration similar filmsincreased the thermal electromotive force, the increase being now in the order lead,tin, and copper, in which the corrosive action might be expected to be promoted by these films.
An attempt was made to calibrate the thermal electromotive forces in terms of temperature.
Modification of Silica Replica Technique for Study of Biological Membranes and Application of Rotary Condensation in Electron Microscopy20(1949); http://dx.doi.org/10.1063/1.1698372View Description Hide Description
A modified silica replica technique for electron microscopy has been developed involving direct condensation of silica upon the object. For improvement of the silica replica technique in general, the following new procedures are introduced:
(a) The undesirable effects of brittleness of silica replica is largely obviated by use of a thin plastic supporting film.
(b) A working method is introduced which completely avoids manual manipulation of the unsupported replicas.
(c) A better definition of small structural elements of the specimen is obtained when a new method of condensing metallic vapors on a rotating replica is introduced; when compared to the usual shadowcasting, an improved two‐dimensional definition of the objects is achieved; also the combined use of rotary condensation and shadowcasting provides a better general definition of objects than would be obtainable by either method alone.
Reproducibility of the morphology of crystals and particles by the modified silica replica is demonstrated with the electron micrographs.
20(1949); http://dx.doi.org/10.1063/1.1698373View Description Hide Description
An automatic method is proposed for recording systematically the density distributions resulting from all permutations of sign of a given set of Fourier coefficients.
It is suggested that the device could be used in conjunction with analog computers of the type now being built by Pepinski, and relay circuits for this purpose are given.
20(1949); http://dx.doi.org/10.1063/1.1698374View Description Hide Description
A simple systematic method, based on a theorem stated and proven here, is presented of obtaining a formula of the form z = f(x, y) from an experimental curve (Fig. 1) in which y is kept constant and a second experimental curve (Fig. 2) in which x is kept constant. Questions of the sufficiency of the original data are automatically answered by this method. Several examples are given to illustrate the procedure explicitly.
20(1949); http://dx.doi.org/10.1063/1.1698375View Description Hide Description
Exact, explicit expressions are obtained for the field of a radiating periodic point dipole located in medium 1 at the center of a spherical shell of medium 2, which is bounded by medium 3. The values of ε, μ, σ and the shell radii are arbitrary. The reflected and transmitted fields are examined in various special cases. When the shell radii become infinite, the fields agree with those obtained by R. K. Luneberg for the case of plane waves normally incident on a flat plate of medium 2 between media 1 and 3. When the radii are infinite and media 2 and 3 identical, the Fresnel formulas for normal incidence of plane waves on a plane interface between two half‐infinite media are obtained. When the radii are finite, media 2 and 3 identical and μ1 = μ3, the results of C. T. Tai are obtained. For finite radii, media 1 and 3 identical, and shell thickness small compared to a wave‐length, the fields check the approximate results of J. B. Keller for the fields reflected and transmitted by a thin shell of any shape, when his results are specialized to the present case.
20(1949); http://dx.doi.org/10.1063/1.1698376View Description Hide Description
For a plane dielectric sheet near a receiving antenna the power varies sinusoidally with distance, having a half‐wave period and an average proportional to the transmission coefficient. This is explained by assuming an equivalent antenna reflection, an assumption checked by special experiments. The theory includes phase and arbitrary incidence, for which the wave may be elliptically polarized. The reciprocal of received amplitude vs.polarization then gives a similar ellipse with 90° shift in orientation. Corresponding theory for transmitters shows that the reflection varies sinusoidally with half‐wave period and with average proportional to the reflection coefficient. For a distant sheet there is inverse‐distance attenuation. The effect of initial antenna mismatch is fully investigated. For any cylindrical surface, including corrugations or a plane, amplitude reflection varies with angle as the secondary power pattern. With a paraboloidal antenna of radius a at wave‐length λ, the reflection from a circular cylinder of radius ρ and reflection coefficientR is 6R(λp)½/13a; that from a narrow strip of width 2aq is (7Rq/80)(15−10q 2+3q 4); that from a narrow strip distant aq′ from the axis is proportional to R(1−q′ 2)4. If the initial mismatch gives a reflection r, and if the surfaces make an angle θ with the axis, the above are multiplied by (1−r 2) exp (4θ2 ln2/w) where w is the antennapattern half‐width. Corresponding results are given for corrugated surfaces and series of strips. Applications to radomes are discussed, and to pressurizing seal design and tolerances. All results are extensively verified experimentally.
20(1949); http://dx.doi.org/10.1063/1.1698381View Description Hide Description