Volume 16, Issue 8, 01 August 1945
Index of content:
16(1945); http://dx.doi.org/10.1063/1.1707612View Description Hide Description
The problem of impedance measurement at centimeter wave‐lengths is discussed with special reference to the practical aspects. The description of the various transmission‐line methods available is supplemented by a detailed account of the apparatus and procedure used to determine the input impedance of a symmetrical antenna.
16(1945); http://dx.doi.org/10.1063/1.1707613View Description Hide Description
Following a critical discussion of the problems of theory and experiment and their coordination, experimentally determined curves for the impedance of a symmetrical, cylindrical antenna terminating a two‐wire line are given and interpreted. The conclusion is reached that the impedance of an antenna, especially near anti‐resonance, depends in a large measure on the proximity and the nature of the input terminals.
16(1945); http://dx.doi.org/10.1063/1.1707614View Description Hide Description
The total secondary emission of Pyrex glass is measured by a method utilizing the conductivity of the heated glass to carry the current. Necessary precautions for this type of measurement are pointed out. The secondary emission is measured over the bombarding voltage range of 50 to 10,000 volts. No temperature variation of secondary emission is observed.
16(1945); http://dx.doi.org/10.1063/1.1707615View Description Hide Description
Fibrils from certain molluscan muscles, in particular the adductor muscles of the clam Venus mercenaria, were examined with the electron microscope and found to possess periodic variations in structure. In order to make these structural variations visible, it was necessary to treat the fibrils with reagents of high electron scattering power (electron stains). Phosphotungstic acid was found to be particularly suitable. This stain combines with specific regions in the fibrils, forming a remarkably regular geometrical pattern of which the most prominent feature is a regular cross striation, representing a fiber‐axis spacing of about 145A. Within each stained band, the stain is more highly concentrated in spots spaced about 193A from center to center across the band. A line drawn through any such spot parallel to the fiber axis passes through other similar spots, spaced five cross bands apart, making the length of the fiber‐axis period precisely five times the fiber‐axis spacing. X‐ray diffraction data obtained by Bear from the intact muscles are compared with the electron microscope observations. The small angle diffractions are in close agreement with those which would be expected from the observed structure except for the magnitude of the lateral spacing. Electron microscope values for this spacing are significantly smaller than the 325A indicated by the x‐ray data, probably as a result of lateral shrinkage in the vacuum‐dried electron microscope specimens. No significant difference in axis spacing has been observed in fibrils isolated from muscles fixed with alcohol in contracted and extended states.
16(1945); http://dx.doi.org/10.1063/1.1707616View Description Hide Description
The Lorentz transformation is applied to the H 01solution of Maxwell's equations used to describe wave propagation in conducting pipes of rectangular cross section. When group velocity for the wave is employed in the transformation, the solution obtained is that one characterizing wave‐guide operation at cut‐off frequency, thus demonstrating that the speed associated with power or signal transmission is group velocity. When phase velocity for the wave is employed in the transformation, and we treat all quantities appearing in the new solution as real, we find that the problem has been reduced to one in magnetostatics and the significance of this solution is pointed out. The use of the inverse Lorentz transformation to find wave guide solutions from known solutions of problems in statics is indicated.
16(1945); http://dx.doi.org/10.1063/1.1707617View Description Hide Description
A practical means for estimating the average performance of an electron microscope is proposed. Some first‐order theory of the operation of an electron‐microscope objective is given to demonstrate the extreme sensitivity of the instrument to the adjustment of the illuminating system. It is shown that by considering the illumination as being produced by a two‐lens system it is possible to explain qualitatively all the effects observed in practice in connection with the illumination of the specimen. Practical information regarding the exact adjustment of the illuminating system is also given. The cause and elimination of multiple images is discussed. Changes in the design of the electron source and the use of an interchangeable aperture in the condenser lens are shown to improve the average performance of the instrument considerably.