Volume 13, Issue 6, 01 June 1942
Index of content:
First Report of the War Policy Committee of The American Institute of Physics, New York, N. Y. May 1, 194213(1942); http://dx.doi.org/10.1063/1.1714876View Description Hide Description
13(1942); http://dx.doi.org/10.1063/1.1714878View Description Hide Description
THE application of the viscosity concept to this technically important field is presented. Discussed first is film lubrication which occurs when the film thickness is greater than 10−5 cm. This mechanism is chiefly controlled by the viscosity η of the liquid. The basic mathematical theory is developed, and it is shown how this adequately describes the experimental facts. Next boundary lubrication is discussed, a type occurring with stable layers less than 10−5 cm thick. This mechanism is essentially a plastic flow within the boundary layer. The chief experimental data are described together with their technical implications.
13(1942); http://dx.doi.org/10.1063/1.1714879View Description Hide Description
X‐ray studies have been made of a number of carbon blacks, prepared under different conditions, and subject to various heat treatments. The patterns were made in evacuated cameras, using CuKα radiation monochromated by reflection from rocksalt. The patterns consist of crystalline reflections (00l), and two‐dimensional lattice reflections (hk). The structure is one of true graphite layers arranged roughly parallel and equidistant, but otherwise completely random. The dimensions within a layer are the same as in graphite; the layer separation is somewhat larger than in graphite. The effect of heat treatment is to increase the size of the parallel layer groups. At graphitization the material changes discontinuously to the crystalline graphitestructure. The usual carbon black is not finely divided graphite. Small angle scattering studies indicate the existence of clusters a few hundred angstroms in size. It is these clusters which are measured by microscope counts, by the electron microscope, and by surface areas, rather than the much smaller parallel layer groups.
13(1942); http://dx.doi.org/10.1063/1.1714880View Description Hide Description
It is shown that high voltage surges are produced in apparatus associated with gas discharge devices when, and only when, the demand for current exceeds the current‐carrying capacity of the gas (or vapor). Cathode spot extinction and sputtering of hot cathodes cause only slight disturbances which do not produce high voltage in practical circuits. Experimental data on surge limits, i.e., the maximum currents that can be carried without surges, are given for four types of mercury vapor tube. These data lead to a single set of values of maximum current density as a function of vapor pressure, which may be used for the safe design of apparatus.
13(1942); http://dx.doi.org/10.1063/1.1714881View Description Hide Description
New developments in an eddycurrent type flaw detector are discussed. Several types of pick‐up units, which contain the exciting coils for inducing eddycurrents in the test material and pick‐up coils for detecting changes in this eddycurrent pattern when a flaw is approached, are described. The electric power for the exciting current is supplied by a vacuum tube power oscillator at a frequency of 500 or 1000 cycles/second. The circuit for detecting the changes in the eddycurrent pattern employs vacuum tube bridge circuits sensitive to both amplitude and phase changes. The use of jigs to guide the pick‐up unit, automatic scanning, and automatic recording is discussed.
13(1942); http://dx.doi.org/10.1063/1.1714882View Description Hide Description
The rapid heating and cooling cycle of the focal area of a high speed radiographic x‐ray tube anode results in short life, although maximum temperatures produced are not excessive. Some objections are found to earlier theoretical investigations of temperature distributions beneath such focal areas. An operational solution is obtained in terms of Bessel functions. From the calculated temperature distribution the mechanism of disruption is explainable.
The Vector Potential and Inductance of a Circuit Comprising Linear Conductors of Different Permeability13(1942); http://dx.doi.org/10.1063/1.1714883View Description Hide Description
Formulas are derived for the vector potential and inductance of a circuit comprised of a long linear cylinder enveloped by a return conductor of eccentric‐annular cross section, the two cylinders and the surrounding medium each being of different permeability. The restriction to very long cylinders rendering the problem two‐dimensional, a familiar scheme of analysis can be employed. A plane cross section is mapped conformally on a rectangle of infinite length and of breadth 2π, the perimeters of the conductors going over into line segments parallel to the short sides. In each of the three resulting rectangular regions of different permeability the vector potential A is expressed as a sum of appropriate infinite double series of trigonometric functions with constant coefficients, these latter being determined in accordance with the known boundary conditions—continuity of the normal component of induction and of the tangential component of field intensity on the perimeters of the conductors. The magnetic field energy is obtained by evaluating W=½∫SwAdS; whence the inductance follows from W=½LI 2. The resulting expression for the inductance yields known formulas for certain special cases of the general problem, thus verifying the analysis. As the symbol for the vector potential is capable of physical reinterpretation as temperature, the foregoing analysis also furnishes solutions to certain heat problems of technical interest.
13(1942); http://dx.doi.org/10.1063/1.1714884View Description Hide Description
A new x‐ray tube for diffraction analysis has been produced which affords much greater x‐ray beam intensity than has been possible with previous designs. In addition the purity of the x‐ray spectrum is insured. The new tube employs thin sheets of malleableberyllium, produced by additions of minute amounts of titanium, for windows in the metal tube wall. The low inherent absorption of such windows and their electrical and mechanical sturdiness are factors in the improvement in beam intensity. A shockproof form of the new diffraction analysis tube is also described.
13(1942); http://dx.doi.org/10.1063/1.1714885View Description Hide Description
A machine is described which has been used for determining the vibration properties of various types of rubber mountings used for the isolation of mechanical vibrations. The vibrations are excited by oppositely rotating eccentric weights and are recorded on a tape. It was found that resonance curves for various types of rubber vibrations such as shear, torsion, and compression are, except for some minor deviations, adequately accounted for by the equation which contains a damping term proportional to the velocity and inversely proportional to the frequency. The apparent static modulus of rubber in compression depends upon the shape of the piece, more explicitly, on the ratio of load area to the free area. The same is true for the dynamic modulus. It was found that pieces of similar shape have identical values for the dynamic modulus and also for the internal friction. Some curves are given showing the dependence of dynamic modulus and internal friction on the shape factor. The dynamic stiffness shows the largest deviation from the static values at high shape factors. The internal friction has a linear relationship to the dynamic modulus when the shape factor is varied. This results in damping which is practically independent of the shape. The effect of temperature on the vibration properties of mounting stocks for a range −10°F to 160°F was studied and curves are included showing the temperature dependence in this range.