Volume 14, Issue 7, 01 July 1943
Index of content:
14(1943); http://dx.doi.org/10.1063/1.1714992View Description Hide Description
Experiments are described for investigating the interpretation of polystyrene‐silica surface replicas. It is demonstrated on a polished and etched stainless steel 18–8 specimen that the light microscope and electron microscope pictures are strikingly similar with corresponding regions easily recognized. The great value of stereo‐pictures in determining relative surface elevations is demonstrated with a comparison of the topography as shown by the light microscope using oblique illumination. The question of resolution is discussed and a method of quantitatively determining the practical limiting resolution of replicas is described. A shape limitation factor, α, is defined and evaluated for both silica and Formvar replicas. With α = 1 being perfect reproduction, the values obtained were α∼1.3 for silica and α∼3.8 for Formvar. The value α∼3.8 is valid only for the surface employed in its determination while the value of α∼1.3 is realized on any surface to which the polystyrene‐silica method can be applied.
14(1943); http://dx.doi.org/10.1063/1.1714993View Description Hide Description
Experiments were conducted to determine the variation of time lag of spark breakdown as a function of gas pressure. All tests were made in pre‐purified nitrogen using 1.250‐inch diameter spheres of copper,aluminum and nickel with a gap spacing of 0.0125 inch. The voltage applied to the gap was in the form of a surge which reached its maximum value in one quarter of a microsecond and which decayed slowly to 97 percent of its maximum value in 5000 microseconds. Most of the recorded lags lay in the range from 5 to 5000 microseconds. The maximum value of the applied voltage was varied from 14 to 128 percent above the static breakdown value, while the gas pressure was varied from 7 to 100 pounds per square inch absolute. The most probable time lag was found to vary greatly with pressure, changing rather abruptly from a large value (10−2 sec.) to a small value (10−5−10−6 sec.) at certain critical conditions of pressure and overvoltage. The change appears to occur under conditions at which the ions normally present in the gas begin to produce free electrons by negative‐ion disintegration, or by positive‐ion bombardment of the cathode. The distribution of time lags was found to vary with cathode material and condition of cathode surface. The nickel spheres quickly developed a heavy coating when sparked in nitrogen. The copper and aluminum spheres became only slightly discolored. The time lag distribution curves for copper and aluminum were of the same type, but different from that for nickel. The observations suggest methods for speeding the action of spark plugs and protective gaps, as well as for increasing the reliability of gas‐insulated apparatus. Apparatus operated below the critical conditions can withstand surges which on the average are one thousand times as long as those which would cause the apparatus to fail when operated in the short time lag region. Similarly, a protective gap or spark plug which is to operate on very short surges can be made to break down more consistently by operating at an overvoltage and a gas pressure in the short time lag region.
14(1943); http://dx.doi.org/10.1063/1.1714994View Description Hide Description
The high resolving power attained with the electron microscope has extended the field of pigment microscopy to cover particles falling within the range of 0.20–0.004 micron. High resolving power also brings into view many particles that would otherwise be invisible, and shows definite shape in particles that with optical instruments appear simply as diffraction disks.
14(1943); http://dx.doi.org/10.1063/1.1714995View Description Hide Description
Formulas are developed for the free periods and damping factors of high voltage surge‐generator circuits in discharge that contain a large number of meshes. The approximations used are based on an actual case, a generator in the High Voltage Laboratory of the National Bureau of Standards.
14(1943); http://dx.doi.org/10.1063/1.1714996View Description Hide Description
Measurements of the thixotropic breakdown of structure with time of agitation were performed at different rates of shear on a number of pigment suspensions and on various oils in a rotational viscometer. The product of rate of breakdown in thixotropic structure and time of agitation at any rate of shear was found to be a constant for each material. It is called ``the time coefficient of thixotropic breakdown'' and is independent of the rate of shear, which is applied to agitate the sample. The time coefficient of thixotropic breakdown is measured in absolute units, having the dimensions of viscosity (dynes sec./cm2). The equilibrium time—the time necessary to break the thixotropic structure down to its minimum at a specified rate of shear—was also found to be independent of the rate of shear applied while agitating.
14(1943); http://dx.doi.org/10.1063/1.1714997View Description Hide Description
The relation arises in certain resonant cavity problems having cylindrical symmetry. The first roots of this relation are presented here as a function of k for n = 1, 2, 3, 4. The M'Mahon relation does not allow calculation of the first roots despite statements to the contrary in several places. It is shown that the functions have relative maxima at x = n except for n = 0.
14(1943); http://dx.doi.org/10.1063/1.1714998View Description Hide Description
The equivalence between the torsional oscillations of a general dynamical system and those of a certain electrical circuit is considered. This equivalence makes it possible to compute the behavior of the dynamical system by making use of some well‐known results of electrical circuit theory. It also makes it possible to determine the critical speeds and the response of the mechanical system by simple electrical measurements of the equivalent system. The forced oscillations of a damped multi‐disk system and those of continuous shafts are studied. The results may be applied by a mere change in notation to the analysis of the longitudinal oscillations of linear spring and mass systems as well as to that of the longitudinal oscillations of prismatic bars.