Volume 12, Issue 10, 01 October 1941
Index of content:
12(1941); http://dx.doi.org/10.1063/1.1712858View Description Hide Description
THE extent to which viscous processes can account for the absorption of freely traveling acoustical waves in gases is shown. Considerable divergence between theory and experimental data is found for polyatomic gases, and an explanation of this divergence on the basis of intramolecular vibrations is presented.
12(1941); http://dx.doi.org/10.1063/1.1712859View Description Hide Description
IN this chapter a technical application of gaseous viscosity in the field of room acoustics is given. After a brief introduction into room acoustics, the absorbing property of sound‐absorbing materials is explained on the basis of viscous processes in the pores of the material. Experimental data are in fair agreement with this explanation. It is even possible to derive the characteristics required for the most efficient absorbent, important from the standpoint of their manufacture.
12(1941); http://dx.doi.org/10.1063/1.1712860View Description Hide Description
Electron microscopes using accelerating potentials of from 30 to 100 kilovolts cannot be employed satisfactorily for the examination of specimens whose thickness is greater than one‐half micron (in the case of organic material). In order to investigate the possibility of applying the high resolving power of the electron microscope to problems involving somewhat thicker specimens, an instrument has been constructed which uses electrons accelerated by voltages up to 300 kilovolts. A commercial electron microscope has been adapted for the purpose by the modification of the electron gun and projection lens, and by the addition of a new high voltage generator. The essential features of the new design and some of the problems encountered in the testing of the equipment are described. Comparison micrographs of test objects taken with 50‐ to 250‐kilovolt electrons demonstrate the increase in penetrating power obtained at the higher potentials. An electron micrograph of a biological section, one to two microns thick, taken with 200‐kilovolt electrons shows a promising amount of the internal structure.
12(1941); http://dx.doi.org/10.1063/1.1712861View Description Hide Description
Studies have been made of the stick‐slip behavior recently investigated by Bowden and collaborators. The friction apparatus consists essentially of a table which can be made to rotate at a very low velocity and a slider elastically supported at the center of rotation of the moving surface. Examples are shown of the stick‐slip process with several combinations of metals at various speeds and loads. Motion pictures of the slider and table have been made in order to study possible slipping during the stick phase. Temperature records of the stick‐slip cycle indicate that the temperature flash is confined exclusively to the slip period and in no case studied exceeds 50°C above ambient. Traces made of the slip phase with very high camera speeds permit an analysis of the variation of kinetic friction with velocity. Thus both static and kinetic friction data can be obtained from a single stick‐slip cycle.
12(1941); http://dx.doi.org/10.1063/1.1712862View Description Hide Description
Taylor and Doran have found that in stabilized glasses the total elastic adjustment under a given load may be represented as the sum of several terms of the type , and have drawn the conclusion that there are several sizes of complex silicate ions in glass which respond independently and with different rates to applied stress.
Hopkinson proposed that there are several independent ``elements'' responding to the application of an electric field to a glass condenser, and von Schweidler showed that the anomalous chargingcurrent accompanying dielectric polarization is the sum of several terms of the type . This expression is identical in form with that for elasticpolarization if the rate constant k is replaced by a characteristicrelaxation time constant 1/T. The dielectric phenomena thus appear to support the concept of independently acting ions in glass. This idea is in harmony with the x‐ray evidence of a random ionic network, and with other physico‐chemical evidence.
An interpretation of memory phenomena such as reversal effects in glass and rubber has been offered. Such reversals appear to be due to the superposition of two or more independent operations with different rate constants and directions.
12(1941); http://dx.doi.org/10.1063/1.1712863View Description Hide Description
It is proposed that the thicknesses of electron microscope objects be determined by measuring the diminution in intensity of the electron beam caused by the object. Since this method would only be applied to specimens so thin that multiple scattering can be neglected, one need know only the total cross section for single scattering of electrons outside the aperture angle of the electron microscope objective. These cross sections are calculated for fast electrons (energies greater than 10,000 ev) by means of the Born approximation for several cases of practical interest, and the results are applied to some experimental observations.