Index of content:
Volume 3, Issue 2, 01 August 1932
3(1932); http://dx.doi.org/10.1063/1.1745084View Description Hide Description
A brief critical summary of the principal contributions published on the theory of the subject is presented, followed by the report of a series of experiments designed to test the methods which have been proposed. It is concluded that while relative measurements can be made with sufficient accuracy, absolute values cannot be relied on as yet. The results emphasize the necessity for careful attention to such details as the construction of the camera, the preparation of the sample, and the characteristics of the x‐ray film and of the microphotometer.
3(1932); http://dx.doi.org/10.1063/1.1745085View Description Hide Description
For several years, it has been well known that, in the natural vibration of a piezoelectric oscillating crystal, there is a certain period proportional only to the thickness, but, so far as we are aware, there has been no concrete explanation for this vibration nor how its period is connected with the physical constants of the medium. The present paper shows that such a vibration, named thickness vibration for brevity, is due to the standing wave produced by interference of plane waves incident to and reflected from the plane boundary surfaces of the medium, and verifies the theoretical results by several examples. There are three normal modes in the thickness vibration, and the corresponding frequencies are given in first approximation by a representative formula: P/2π = (q/2a) (c/ρ)1/2, where q is any integer, a the thickness of the plate, ρ the density and c a certain adiabatic elastic constant depending upon the orientation of the plate with respect to the crystallographic axes of the medium. It is not always possible to excite all normal modes of vibration piezoelectrically. The electrically measured natural frequency of the thickness vibration is always a little higher than that calculated by the formula. Adiabatic elastic constants of piezoelectric crystals may be determined in first approximation by measurement of the frequencies of thickness vibrations of plates prepared from given crystals.
3(1932); http://dx.doi.org/10.1063/1.1745086View Description Hide Description
Bidwell showed that the resistance‐temperature law for certain variable conductors (oxides, etc.) was of the form ρ = AeQ/RT+aT . Plotted in the form (1/ρ)(dρ/dT) = Q/RT 2−a he obtained for Fe2O3 straight lines with a break near the recalescence point, the two lines having the same y intercept but different slopes. This law was also followed by metallic germanium. The present paper extends this work to zinc oxide and beryllium oxide. Beryllium oxide yields a straight line showing a transition at 750°C. Zinc oxide yields two straight lines with a transition in the interval 250°C—500°C. The two zinc oxide lines have different slopes but the same y intercepts. With successive heatings to 1050°C there occurs continued decreases in the slope of the line for the 500°C—1000°C range but no change in the y intercept. The line for the range 0°C—250°C changes on successive heatings to 1050°C and finally stabilizes to a value of slope and intercept which repeats on succeeding runs. On the suggesion that the indicated transformation was due to impurity, new material of special purity prepared by the New Jersey Zinc Company was studied. The behavior of this material was found to agree closely with that of the earlier specimen but gave a more clean cut transformation.
3(1932); http://dx.doi.org/10.1063/1.1745087View Description Hide Description
Recent published results of studies on the flow of liquids through porous media under the action of gravity have shown wide disagreement in both the formulation and interpretation of the problem. A new attack on the problem has been carried out and has led to unambiguous answers to the questions of interest. Experiments on a radial sector of sand show that the old Dupuit formula of 1863 stating that the fluid outflow is proportional to the square of the differences in the fluid heights in the sand is exact within experimental error provided the fluid heights are replaced by fluid heads as measured at the sand bottom. Both the pressure distribution formula and the integrated expression for the fluid outflow are verified in detail. The cases where an added pressure head is superposed upon the gravity flow, heretofore not mentioned in the literature, behaves both with regard to its pressure distribution and fluid outflow as a direct superposition of the simple gravity flow and pure radial flow systems. A semi‐quantitative treatment is given for the flow in the capillary zone above the main fluid body. It is necessary under certain experimental conditions to correct for the added flow carried by the capillary layer in addition to that induced by the gravity drive. In most practical cases the capillary flow may be ignored. A theoretical discussion is given of the conditions at the free surface of a gravity flow system and it is shown that nowhere on it can the slope exceed 45°. The results on the radial flow experiments are also generalized to give a method for treating general drives.
3(1932); http://dx.doi.org/10.1063/1.1745082View Description Hide Description
An expression has been derived on the basis of the Maxwell‐Boltzmann distribution and Poisson's equation for the tangential surface conductance of an electrolyte in contact with a charged plane surface. One part of this expression is analogous to the result given by Smoluchowski for a Helmholtz double layer, while the second part arises from the movement of the ions in the diffuse ion cloud under the applied electric field. The theory shows qualitative agreement with the data of Briggs for the conductance of univalent chloride solutions at a cellulose surface. For all but the most dilute solutions, the Smoluchowski term is negligible and the surface conductance is proportional to the charge density on the surface. The theory does not agree with the data for different anions and it is suggested that the discrepancy may be due to absorption phenomena. There may be a movement of the adsorbed ions or of a diffuse ion cloud resulting from the presence of adsorption potentials at a distance from the interface.